<PAGE>   1
              As filed with the Securities and Exchange Commission
                                On March 9, 1998

                              Registration No. 33-

--------------------------------------------------------------------------------
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                    FORM S-8
                             REGISTRATION STATEMENT
                       AND POST-EFFECTIVE AMENDMENT NO. 1
                     TO REGISTRATION STATEMENT NO. 33-73544
                       AND POST-EFFECTIVE AMENDMENT NO. 2
                     TO REGISTRATION STATEMENT NO. 33-41534

                                      UNDER

                           THE SECURITIES ACT OF 1933

                                 IMMUNOGEN, INC.
              ----------------------------------------------------
             (Exact name of registrant as specified in its charter)

       Massachusetts                                        04-2726691
---------------------------------                   --------------------------
(State or other jurisdiction of                          (I.R.S. Employer
 incorporation or organization)                         Identification No.)

              333 Providence Highway, Norwood, Massachusetts 02062
              ----------------------------------------------------
                    (Address of principal executive offices)

                   IMMUNOGEN, INC. RESTATED STOCK OPTION PLAN
                   ------------------------------------------
                            (Full title of the plan)

                                 Mitchel Sayare
                            Chairman of the Board and
                             Chief Executive Officer
                                 ImmunoGen, Inc.
                             333 Providence Highway
                          Norwood, Massachusetts 02062
              ----------------------------------------------------
                     (Name and address of agent for service)

                                 (781) 769-4242
              ----------------------------------------------------
          (Telephone number, including area code, of agent for service)


<PAGE>   2



                         CALCULATION OF REGISTRATION FEE
<TABLE>
<CAPTION>
--------------------------------------------------------------------------------
                                    Proposed      Proposed
Title of                            maximum       maximum
securities          Amount          offering      aggregate         Amount of
to be               to be           price per     offering          registration
registered          registered(1)   unit(4)       price(4)          fee
--------------------------------------------------------------------------------
<S>                 <C>              <C>         <C>                  <C>   
Common Stock,       1,463,372 (2)    $2.643      $3,867,692           $1,141
$.01 par value
Common Stock,         361,628 (3)     1.969         712,046              210
$.01 par value      ---------                    ----------           ------
   TOTAL            1,825,000                    $4,579,738           $1,351
                    ---------                    ----------           ------
--------------------------------------------------------------------------------
</TABLE>

(1)  The number of shares of common stock, par value $.01 per share (the "Common
     Stock"), consists of the aggregate number of additional shares not
     previously registered which may be sold upon the exercise of options which
     have been granted and/or may hereafter be granted under the ImmunoGen, Inc.
     Restated Stock Option Plan (the "Plan"). The maximum number of shares which
     may be sold upon the exercise of such options granted under the Plan is
     subject to adjustment in accordance with certain anti-dilution and other
     provisions of the Plan. Accordingly, pursuant to Rule 416 under the
     Securities Act of 1933, as amended (the "Securities Act"), this
     Registration Statement covers, in addition to the number of shares stated
     above, an indeterminable number of shares which may be subject to grant or
     otherwise issuable after the operation of any such anti-dilution and other
     provisions.

(2)  Consists of the aggregate number of shares of Common Stock which may be
     sold upon the exercise of options which previously have been granted under
     the Plan.

(3)  Consists of the aggregate number of shares of Common Stock which may be
     sold upon the exercise of options which may hereafter be granted under the
     Plan.

(4)  This calculation is made solely for the purpose of determining the
     registration fee pursuant to the provisions of Rule 457(h) under the
     Securities Act as follows: (i) in the case of shares of Common Stock which
     may be purchased upon exercise of outstanding options, the fee is
     calculated on the basis of the price at which the options may be exercised;
     and (ii) in the case of shares of Common Stock for which options have not
     yet been granted and the option price of which is therefore unknown, the
     fee is calculated on the basis of the average high and low sales prices per
     share of the Common Stock on the National Market System of the National
     Association of Securities Dealers Automated Quotation System as of a date
     (March 3, 1998) within 5 business days prior to filing this Registration
     Statement.

                                       2
<PAGE>   3

EXPLANATORY NOTE

     This Post-Effective Amendment and Registration Statement relates, in part,
to the registration of 1,825,000 additional shares of Common Stock authorized
for issuance under the Plan. It also constitutes a Post-Effective Amendment with
respect to 1,000,000 and 700,000 previously registered shares of Common Stock
authorized for issuance under the Plan, registered on Registration Statement No.
33-41534 (filed July 3, 1991) and Registration Statement No. 33-73544 (filed
December 27, 1993), respectively. In accordance with the instructional Note to
Part I of Form S-8 as promulgated by the Securities and Exchange Commission, the
information specified by Part I of Form S-8 has been omitted from this
Registration Statement on Form S-8 for offers of Common Stock pursuant to the
Plan. The Prospectus filed as part of the Company's Registration Statement No.
33-41534 and registration statement No. 33-73544 may no longer be used for
reofferings and resales of Common Stock because the information in such
propectuses is no longer accurate.

                                       3

<PAGE>   4




                                     PART II
               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

ITEM 3. INCORPORATION OF DOCUMENTS BY REFERENCE.

     The following documents filed by the Registrant with the Securities and
Exchange Commission (the "Commission") are incorporated herein by reference:

     (a) The Registrant's annual report on Form 10-K (Commission File No.
0-17999) for the fiscal year ended June 30, 1997.

     (b) The Company's quarterly report on Form 10-Q (Commission File No.
0-17999) for the quarter ended September 30, 1997, and Amendment No. 1 to the
Company's quarterly report on Form 10-Q for the quarter ended September 30,
1997, on Form 10-Q/A, filed with the Commission on November 24, 1997. The
Company's quarterly report on Form 10-Q (Commission File No. 0-17999) for the
quarter ended December 31, 1997.

     (c) The Company's current report on Form 8-K for the August 1, 1997 event
announcing a collaboration between the Company's 95%-owned subsidiary and
BioChem Pharma Inc., as filed with the Commission on August 1, 1997; and the
Company's current report on Form 8-K for the December 11, 1997 event announcing
a $3.0 million investment in the Company by Biotechnology Value Fund, L.P. and
affiliates, as filed with the Commission on December 11, 1997.

     (d) The description of the Common Stock included in the Registrant's
Registration Statement on Form 8-A under the Securities Exchange Act of 1934, as
amended (the "Exchange Act"), filed with the Commission on September 22, 1989,
including any amendment or report filed for the purpose of updating such
description.

     All reports and other documents subsequently filed by the Registrant with
the Commission pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Exchange
Act prior to the filing of a post-effective amendment which indicates that all
securities covered by this Registration Statement have been sold or which
deregisters all such securities then remaining unsold shall be deemed to be
incorporated by reference herein and to be a part hereof from the date of the
filing of such reports and documents.

ITEM 4. DESCRIPTION OF SECURITIES.

     Not applicable.

ITEM 5. INTERESTS OF NAMED EXPERTS AND COUNSEL.

     Not applicable.

ITEM 6. INDEMNIFICATION OF DIRECTORS AND OFFICERS.

     Incorporated herein by reference from the Registrant's Registration
Statement on Form S-1 (Registration No. 33-43725), as amended.

                                       4
<PAGE>   5

ITEM 7. EXEMPTION FROM REGISTRATION CLAIMED.

     Not applicable.

ITEM 8. EXHIBITS.

EXHIBIT NO.                   DESCRIPTION

(4.1)    Form of Common Stock Certificate (previously filed as Exhibit 4.2 to
         the Registrant's Registration Statement on Form S-1, Registration No.
         33-31219, as amended, and incorporated herein by reference)

(4.2)    Restated Articles of Organization of the Registrant (previously filed
         as Exhibit 3.1 to the Registrant's Registration Statement on Form S-1,
         Registration No. 33-38883, as amended, and incorporated herein by
         reference)

(4.3)    By-Laws, as amended, of the Registrant (previously filed as Exhibit 3.2
         to the Registrant's annual report on Form 10-K for the fiscal year
         ended June 30, 1990, and incorporated herein by reference)

(5)      Opinion of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C., with
         respect to the legality of the shares being registered

(23.1)   Consent of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.
         (included in opinion of counsel filed as Exhibit 5)

(23.2)   Consent of Coopers & Lybrand

(24)     Power of Attorney to file future amendments (included on the signature
         page of this Registration Statement)

(99)     ImmunoGen, Inc. Restated Stock Option Plan (as amended)

ITEM 9. UNDERTAKINGS.

         (a)      The undersigned Registrant hereby undertakes:

                  (1) To file, during any period in which offers or sales are
         being made, a post-effective amendment to this Registration Statement:

                  (i)      To include any prospectus required by Section
                           10(a)(3) of the Securities Act of 1933;

                  (ii)     To reflect in the prospectus any facts or events
                           arising after the effective date of the Registration
                           Statement (or the most recent post-effective
                           amendment thereof) which, individually or in the
                           aggregate, represent a fundamental change in the
                           information set forth in the Registration Statement.
                           Notwithstanding the foregoing, any increase or
                           decrease in volume of

                                       5
<PAGE>   6


                           securities offered (if the total dollar value of
                           securities offered would not exceed that which was
                           registered) and any deviation from the low or high
                           end of the estimated maximum offering range may be
                           reflected in the form of prospectus filed with the
                           Commission pursuant to Rule 424(b) of Regulation C
                           under the Securites Act of 1933, as amended, if, in
                           the aggregate, the changes in volume and price
                           represent no more than a 20% change in the maximum
                           aggregate offering price set forth in the
                           "Calculation of Registration Fee" table in the
                           effective Registration Statement;

                  (iii)    To include any material information with respect to
                           the plan of distribution not previously disclosed in
                           the Registration Statement or any material change to
                           such information in the Registration Statement;

                  provided, however, that paragraphs (a)(1)(i) and (a)(1)(ii) of
                  this section do not apply if the Registration Statement is on
                  Form S-3, Form S-8 or Form F-3, and the information required
                  to be included in a post-effective amendment by those
                  paragraphs is contained in periodic reports filed with or
                  furnished to the Commission by the Registrant pursuant to
                  Section 13 or Section 15(d) of the Securities Exchange Act of
                  1934 that are incorporated by reference in this Registration
                  Statement.

         (2) That, for the purpose of determining any liability under the
     Securities Act of 1933, each such post-effective amendment shall be deemed 
     to be a new registration statement relating to the securities offered
     therein, and the offering of such securities at that time shall be deemed
     to be the initial bona fide offering thereof.

         (3) To remove from registration by means of a post-effective amendment
     any of the securities being registered which remain unsold at the 
     termination of the offering.

    (b) The undersigned Registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act of 1933, each filing of the
Registrant's annual report pursuant to Section 13(a) or Section 15(d) of the
Securities Exchange Act of 1934 (and, where applicable, each filing of an
employee benefit plan's annual report pursuant to section 15(d) of the
Securities Exchange Act of 1934) that is incorporated by reference in this
Registration Statement shall be deemed to be a new registration statement
relating to the securities offered therein, and the offering of such securities
at that time shall be deemed to be the initial bona fide offering thereof.

    (c) Insofar as indemnification for liabilities arising under the Securities
Act of 1933 may be permitted to directors, officers and controlling persons of
the Registrant pursuant to the foregoing provisions, or otherwise, the
Registrant has been advised that in the opinion of the Securities and Exchange
Commission such indemnification is against public policy as expressed in the
Securities Act of 1933 and is, therefore, unenforceable. In the event that a
claim for indemnification against such liabilities (other than the payment by
the Registrant of expenses incurred or paid by a director, officer or
controlling person of the Registrant in the successful defense of any action,
suit or proceeding) is asserted by such director, officer or controlling person
in connection with the securities being registered, the Registrant will, unless
in the opinion of its counsel the matter has been settled by controlling
precedent, submit to a court of appropriate jurisdiction the question whether
such indemnification by it is against public policy as expressed in the
Securities Act of 1933 and will be governed by the final adjudication of such
issue.

                                       6
<PAGE>   7

                                   SIGNATURES

         Pursuant to the requirements of the Securities Act of 1933, the
Registrant certifies that it has reasonable grounds to believe that it meets all
of the requirements for filing on Form S-8 and has duly caused this Registration
Statement and Post-Effective Amendment No. 1 to Registration Statement No.
33-73544 and Post-Effective Amendment No. 2 to Registration Statement No.
33-41534 to be signed on its behalf by the undersigned, thereunto duly
authorized, in the Town of Norwood, Commonwealth of Massachusetts, on March 6,
1998.

                                                IMMUNOGEN, INC.


                                                 /s/ Mitchel Sayare
                                                --------------------------------
                                                By:  Mitchel Sayare
                                                     Chairman of the Board
                                                     and Chief Executive Officer

         Each person whose signature appears below constitutes and appoints
Mitchel Sayare and Kathleen A. Carroll, and each of them, his true and lawful
attorneys-in-fact and agents, with full power of substitution and resubstitution
in each of them, for him and in his name, place and stead, and in any and all
capacities, to sign any and all amendments (including post-effective amendments)
to this Registration Statement, and to file the same, with all exhibits thereto
and other documents in connection therewith, with the Securities and Exchange
Commission, granting unto said attorneys-in-fact and agents, and each of them,
full power and authority to do and perform each and every act and thing
requisite or necessary to be done in and about the premises, as fully to all
intents and purposes as he might or could do in person, hereby ratifying and
confirming all that said attorneys-in-fact and agents or any of them or their or
his substitute or substitutes may lawfully do or cause to be done by virtue
hereof.

         Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities and on the dates indicated.
<TABLE>
<CAPTION>
<S>                                 <C>                                 <C>
/s/ Mitchel Sayare                  Chairman of the Board               March 6, 1998
--------------------------          of Directors and
Mitchel Sayare                      Chief Executive Officer
                                    (Principal Executive Officer)


/s/ Walter A. Blattler              Executive Vice President,           March 6, 1998            
------------------------            Science and Technology
Walter A. Blattler                  and Director



/s/ Kathleen A. Carroll             Vice President, Finance             March 6, 1998
--------------------------          and Administration and
Kathleen A. Carroll                 Treasurer (Principal Financial
                                    Officer and Principal Accounting
                                    Officer)

/s/ David W. Carter                 Director                            March 6, 1998
--------------------------
David W. Carter

                                    Director                            
--------------------------
Michael R. Eisenson

/s/ Stuart F. Feiner                Director                            March 6, 1998
--------------------------
Stuart F. Feiner

/s/ Donald E. O'Neill               Director                            March 6, 1998
--------------------------
Donald E. O'Neill
</TABLE>

                                       7
<PAGE>   8

                                 IMMUNOGEN, INC.

                                  EXHIBIT INDEX




EXHIBIT NO.                   DESCRIPTION

(4.1)    Form of Common Stock Certificate (previously filed as Exhibit 4.2 to
         the Registrant's Registration Statement on Form S-1, Registration No.
         33-31219, as amended, and incorporated herein by reference)

(4.2)    Restated Articles of Organization of the Registrant (previously filed
         as Exhibit 3.1 to the Registrant's Registration Statement on Form S-1,
         Registration No. 33-38883, as amended, and incorporated herein by
         reference)

(4.3)    By-Laws, as amended, of the Registrant (previously filed as Exhibit 3.2
         to the Registrant's annual report on Form 10-K for the fiscal year
         ended June 30, 1990, and incorporated herein by reference)

(5)      Opinion of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C., with
         respect to the legality of the shares being registered

(23.1)   Consent of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.
         (included in opinion of counsel filed as Exhibit 5)

(23.2)   Consent of Coopers & Lybrand

(24)     Power of Attorney to file future amendments (included on the signature
         page of this Registration Statement)

(99)     ImmunoGen, Inc. Restated Stock Option Plan (as amended)


                                       8
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1
              As filed with the Securities and Exchange Commission
                                On March 9, 1998

                              Registration No. 33-

--------------------------------------------------------------------------------
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                    FORM S-8
                             REGISTRATION STATEMENT
                       AND POST-EFFECTIVE AMENDMENT NO. 1
                     TO REGISTRATION STATEMENT NO. 33-73544
                       AND POST-EFFECTIVE AMENDMENT NO. 2
                     TO REGISTRATION STATEMENT NO. 33-41534

                                      UNDER

                           THE SECURITIES ACT OF 1933

                                 IMMUNOGEN, INC.
              ----------------------------------------------------
             (Exact name of registrant as specified in its charter)

       Massachusetts                                        04-2726691
---------------------------------                   --------------------------
(State or other jurisdiction of                          (I.R.S. Employer
 incorporation or organization)                         Identification No.)

              333 Providence Highway, Norwood, Massachusetts 02062
              ----------------------------------------------------
                    (Address of principal executive offices)

                   IMMUNOGEN, INC. RESTATED STOCK OPTION PLAN
                   ------------------------------------------
                            (Full title of the plan)

                                 Mitchel Sayare
                            Chairman of the Board and
                             Chief Executive Officer
                                 ImmunoGen, Inc.
                             333 Providence Highway
                          Norwood, Massachusetts 02062
              ----------------------------------------------------
                     (Name and address of agent for service)

                                 (781) 769-4242
              ----------------------------------------------------
          (Telephone number, including area code, of agent for service)
2

                         CALCULATION OF REGISTRATION FEE
--------------------------------------------------------------------------------
                                    Proposed      Proposed
Title of                            maximum       maximum
securities          Amount          offering      aggregate         Amount of
to be               to be           price per     offering          registration
registered          registered(1)   unit(4)       price(4)          fee
--------------------------------------------------------------------------------
Common Stock,       1,463,372 (2)    $2.643      $3,867,692           $1,141
$.01 par value
Common Stock,         361,628 (3)     1.969         712,046              210
$.01 par value      ---------                    ----------           ------
   TOTAL            1,825,000                    $4,579,738           $1,351
                    ---------                    ----------           ------
--------------------------------------------------------------------------------
(1)  The number of shares of common stock, par value $.01 per share (the "Common
     Stock"), consists of the aggregate number of additional shares not
     previously registered which may be sold upon the exercise of options which
     have been granted and/or may hereafter be granted under the ImmunoGen, Inc.
     Restated Stock Option Plan (the "Plan"). The maximum number of shares which
     may be sold upon the exercise of such options granted under the Plan is
     subject to adjustment in accordance with certain anti-dilution and other
     provisions of the Plan. Accordingly, pursuant to Rule 416 under the
     Securities Act of 1933, as amended (the "Securities Act"), this
     Registration Statement covers, in addition to the number of shares stated
     above, an indeterminable number of shares which may be subject to grant or
     otherwise issuable after the operation of any such anti-dilution and other
     provisions.

(2)  Consists of the aggregate number of shares of Common Stock which may be
     sold upon the exercise of options which previously have been granted under
     the Plan.

(3)  Consists of the aggregate number of shares of Common Stock which may be
     sold upon the exercise of options which may hereafter be granted under the
     Plan.

(4)  This calculation is made solely for the purpose of determining the
     registration fee pursuant to the provisions of Rule 457(h) under the
     Securities Act as follows: (i) in the case of shares of Common Stock which
     may be purchased upon exercise of outstanding options, the fee is
     calculated on the basis of the price at which the options may be exercised;
     and (ii) in the case of shares of Common Stock for which options have not
     yet been granted and the option price of which is therefore unknown, the
     fee is calculated on the basis of the average high and low sales prices per
     share of the Common Stock on the National Market System of the National
     Association of Securities Dealers Automated Quotation System as of a date
     (March 3, 1998) within 5 business days prior to filing this Registration
     Statement.

                                       2
3

EXPLANATORY NOTE

     This Post-Effective Amendment and Registration Statement relates, in part,
to the registration of 1,825,000 additional shares of Common Stock authorized
for issuance under the Plan. It also constitutes a Post-Effective Amendment with
respect to 1,000,000 and 700,000 previously registered shares of Common Stock
authorized for issuance under the Plan, registered on Registration Statement No.
33-41534 (filed July 3, 1991) and Registration Statement No. 33-73544 (filed
December 27, 1993), respectively. In accordance with the instructional Note to
Part I of Form S-8 as promulgated by the Securities and Exchange Commission, the
information specified by Part I of Form S-8 has been omitted from this
Registration Statement on Form S-8 for offers of Common Stock pursuant to the
Plan. The Prospectus filed as part of the Company's Registration Statement No.
33-41534 and registration statement No. 33-73544 may no longer be used for
reofferings and resales of Common Stock because the information in such
propectuses is no longer accurate.

                                       3
4

                                     PART II
               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

ITEM 3. INCORPORATION OF DOCUMENTS BY REFERENCE.

     The following documents filed by the Registrant with the Securities and
Exchange Commission (the "Commission") are incorporated herein by reference:

     (a) The Registrant's annual report on Form 10-K (Commission File No.
0-17999) for the fiscal year ended June 30, 1997.

     (b) The Company's quarterly report on Form 10-Q (Commission File No.
0-17999) for the quarter ended September 30, 1997, and Amendment No. 1 to the
Company's quarterly report on Form 10-Q for the quarter ended September 30,
1997, on Form 10-Q/A, filed with the Commission on November 24, 1997. The
Company's quarterly report on Form 10-Q (Commission File No. 0-17999) for the
quarter ended December 31, 1997.

     (c) The Company's current report on Form 8-K for the August 1, 1997 event
announcing a collaboration between the Company's 95%-owned subsidiary and
BioChem Pharma Inc., as filed with the Commission on August 1, 1997; and the
Company's current report on Form 8-K for the December 11, 1997 event announcing
a $3.0 million investment in the Company by Biotechnology Value Fund, L.P. and
affiliates, as filed with the Commission on December 11, 1997.

     (d) The description of the Common Stock included in the Registrant's
Registration Statement on Form 8-A under the Securities Exchange Act of 1934, as
amended (the "Exchange Act"), filed with the Commission on September 22, 1989,
including any amendment or report filed for the purpose of updating such
description.

     All reports and other documents subsequently filed by the Registrant with
the Commission pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Exchange
Act prior to the filing of a post-effective amendment which indicates that all
securities covered by this Registration Statement have been sold or which
deregisters all such securities then remaining unsold shall be deemed to be
incorporated by reference herein and to be a part hereof from the date of the
filing of such reports and documents.

ITEM 4. DESCRIPTION OF SECURITIES.

     Not applicable.

ITEM 5. INTERESTS OF NAMED EXPERTS AND COUNSEL.

     Not applicable.

ITEM 6. INDEMNIFICATION OF DIRECTORS AND OFFICERS.

     Incorporated herein by reference from the Registrant's Registration
Statement on Form S-1 (Registration No. 33-43725), as amended.

                                       4
5

ITEM 7. EXEMPTION FROM REGISTRATION CLAIMED.

     Not applicable.

ITEM 8. EXHIBITS.

EXHIBIT NO.                   DESCRIPTION

(4.1)    Form of Common Stock Certificate (previously filed as Exhibit 4.2 to
         the Registrant's Registration Statement on Form S-1, Registration No.
         33-31219, as amended, and incorporated herein by reference)

(4.2)    Restated Articles of Organization of the Registrant (previously filed
         as Exhibit 3.1 to the Registrant's Registration Statement on Form S-1,
         Registration No. 33-38883, as amended, and incorporated herein by
         reference)

(4.3)    By-Laws, as amended, of the Registrant (previously filed as Exhibit 3.2
         to the Registrant's annual report on Form 10-K for the fiscal year
         ended June 30, 1990, and incorporated herein by reference)

(5)      Opinion of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C., with
         respect to the legality of the shares being registered

(23.1)   Consent of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.
         (included in opinion of counsel filed as Exhibit 5)

(23.2)   Consent of Coopers & Lybrand

(24)     Power of Attorney to file future amendments (included on the signature
         page of this Registration Statement)

(99)     ImmunoGen, Inc. Restated Stock Option Plan (as amended)

ITEM 9. UNDERTAKINGS.

         (a)      The undersigned Registrant hereby undertakes:

                  (1) To file, during any period in which offers or sales are
         being made, a post-effective amendment to this Registration Statement:

                  (i)      To include any prospectus required by Section
                           10(a)(3) of the Securities Act of 1933;

                  (ii)     To reflect in the prospectus any facts or events
                           arising after the effective date of the Registration
                           Statement (or the most recent post-effective
                           amendment thereof) which, individually or in the
                           aggregate, represent a fundamental change in the
                           information set forth in the Registration Statement.
                           Notwithstanding the foregoing, any increase or
                           decrease in volume of

                                       5
6

                           securities offered (if the total dollar value of
                           securities offered would not exceed that which was
                           registered) and any deviation from the low or high
                           end of the estimated maximum offering range may be
                           reflected in the form of prospectus filed with the
                           Commission pursuant to Rule 424(b) of Regulation C
                           under the Securites Act of 1933, as amended, if, in
                           the aggregate, the changes in volume and price
                           represent no more than a 20% change in the maximum
                           aggregate offering price set forth in the
                           "Calculation of Registration Fee" table in the
                           effective Registration Statement;

                  (iii)    To include any material information with respect to
                           the plan of distribution not previously disclosed in
                           the Registration Statement or any material change to
                           such information in the Registration Statement;

                  provided, however, that paragraphs (a)(1)(i) and (a)(1)(ii) of
                  this section do not apply if the Registration Statement is on
                  Form S-3, Form S-8 or Form F-3, and the information required
                  to be included in a post-effective amendment by those
                  paragraphs is contained in periodic reports filed with or
                  furnished to the Commission by the Registrant pursuant to
                  Section 13 or Section 15(d) of the Securities Exchange Act of
                  1934 that are incorporated by reference in this Registration
                  Statement.

         (2) That, for the purpose of determining any liability under the
     Securities Act of 1933, each such post-effective amendment shall be deemed 
     to be a new registration statement relating to the securities offered
     therein, and the offering of such securities at that time shall be deemed
     to be the initial bona fide offering thereof.

         (3) To remove from registration by means of a post-effective amendment
     any of the securities being registered which remain unsold at the 
     termination of the offering.

    (b) The undersigned Registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act of 1933, each filing of the
Registrant's annual report pursuant to Section 13(a) or Section 15(d) of the
Securities Exchange Act of 1934 (and, where applicable, each filing of an
employee benefit plan's annual report pursuant to section 15(d) of the
Securities Exchange Act of 1934) that is incorporated by reference in this
Registration Statement shall be deemed to be a new registration statement
relating to the securities offered therein, and the offering of such securities
at that time shall be deemed to be the initial bona fide offering thereof.

    (c) Insofar as indemnification for liabilities arising under the Securities
Act of 1933 may be permitted to directors, officers and controlling persons of
the Registrant pursuant to the foregoing provisions, or otherwise, the
Registrant has been advised that in the opinion of the Securities and Exchange
Commission such indemnification is against public policy as expressed in the
Securities Act of 1933 and is, therefore, unenforceable. In the event that a
claim for indemnification against such liabilities (other than the payment by
the Registrant of expenses incurred or paid by a director, officer or
controlling person of the Registrant in the successful defense of any action,
suit or proceeding) is asserted by such director, officer or controlling person
in connection with the securities being registered, the Registrant will, unless
in the opinion of its counsel the matter has been settled by controlling
precedent, submit to a court of appropriate jurisdiction the question whether
such indemnification by it is against public policy as expressed in the
Securities Act of 1933 and will be governed by the final adjudication of such
issue.

                                       6
7

                                   SIGNATURES

         Pursuant to the requirements of the Securities Act of 1933, the
Registrant certifies that it has reasonable grounds to believe that it meets all
of the requirements for filing on Form S-8 and has duly caused this Registration
Statement and Post-Effective Amendment No. 1 to Registration Statement No.
33-73544 and Post-Effective Amendment No. 2 to Registration Statement No.
33-41534 to be signed on its behalf by the undersigned, thereunto duly
authorized, in the Town of Norwood, Commonwealth of Massachusetts, on March 6,
1998.

                                                IMMUNOGEN, INC.

                                                 /s/ Mitchel Sayare
                                                --------------------------------
                                                By:  Mitchel Sayare
                                                     Chairman of the Board
                                                     and Chief Executive Officer

         Each person whose signature appears below constitutes and appoints
Mitchel Sayare and Kathleen A. Carroll, and each of them, his true and lawful
attorneys-in-fact and agents, with full power of substitution and resubstitution
in each of them, for him and in his name, place and stead, and in any and all
capacities, to sign any and all amendments (including post-effective amendments)
to this Registration Statement, and to file the same, with all exhibits thereto
and other documents in connection therewith, with the Securities and Exchange
Commission, granting unto said attorneys-in-fact and agents, and each of them,
full power and authority to do and perform each and every act and thing
requisite or necessary to be done in and about the premises, as fully to all
intents and purposes as he might or could do in person, hereby ratifying and
confirming all that said attorneys-in-fact and agents or any of them or their or
his substitute or substitutes may lawfully do or cause to be done by virtue
hereof.

         Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities and on the dates indicated.
/s/ Mitchel Sayare                  Chairman of the Board               March 6, 1998
--------------------------          of Directors and
Mitchel Sayare                      Chief Executive Officer
                                    (Principal Executive Officer)

/s/ Walter A. Blattler              Executive Vice President,           March 6, 1998            
------------------------            Science and Technology
Walter A. Blattler                  and Director

/s/ Kathleen A. Carroll             Vice President, Finance             March 6, 1998
--------------------------          and Administration and
Kathleen A. Carroll                 Treasurer (Principal Financial
                                    Officer and Principal Accounting
                                    Officer)

/s/ David W. Carter                 Director                            March 6, 1998
--------------------------
David W. Carter

                                    Director                            
--------------------------
Michael R. Eisenson

/s/ Stuart F. Feiner                Director                            March 6, 1998
--------------------------
Stuart F. Feiner

/s/ Donald E. O'Neill               Director                            March 6, 1998
--------------------------
Donald E. O'Neill
7
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                                 IMMUNOGEN, INC.

                                  EXHIBIT INDEX

EXHIBIT NO.                   DESCRIPTION

(4.1)    Form of Common Stock Certificate (previously filed as Exhibit 4.2 to
         the Registrant's Registration Statement on Form S-1, Registration No.
         33-31219, as amended, and incorporated herein by reference)

(4.2)    Restated Articles of Organization of the Registrant (previously filed
         as Exhibit 3.1 to the Registrant's Registration Statement on Form S-1,
         Registration No. 33-38883, as amended, and incorporated herein by
         reference)

(4.3)    By-Laws, as amended, of the Registrant (previously filed as Exhibit 3.2
         to the Registrant's annual report on Form 10-K for the fiscal year
         ended June 30, 1990, and incorporated herein by reference)

(5)      Opinion of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C., with
         respect to the legality of the shares being registered

(23.1)   Consent of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.
         (included in opinion of counsel filed as Exhibit 5)

(23.2)   Consent of Coopers & Lybrand

(24)     Power of Attorney to file future amendments (included on the signature
         page of this Registration Statement)

(99)     ImmunoGen, Inc. Restated Stock Option Plan (as amended)

                                       8