<PAGE>


              As filed with the Securities and Exchange Commission
                               On January 5, 2001

                              Registration No. 33-


                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                    FORM S-8
                             REGISTRATION STATEMENT

                                      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.)


                128 SIDNEY STREET, CAMBRIDGE, MASSACHUSETTS 02139
                -------------------------------------------------
                    (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.
                                128 Sidney Street
                         Cambridge, Massachusetts 02139
                         ------------------------------
                     (NAME AND ADDRESS OF AGENT FOR SERVICE)

                                 (617) 995-2500
                                 --------------
          (TELEPHONE NUMBER, INCLUDING AREA CODE, OF AGENT FOR SERVICE)

                  Please send copies of all communications to:

                            Jonathan L. Kravetz, Esq.
                           Mintz, Levin, Cohn, Ferris,
                             Glovsky and Popeo, P.C.
                              One Financial Center
                                Boston, MA 02111
                                 (617) 542-6000


                                       1


<PAGE>



                         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,
$.01 par value      472,163 (2)         $ 8.14        $ 3,843,407   $  960.85


Common Stock,
$.01 par value      852,837 (3)          20.50         17,483,158   $4,370.79
                  ---------


   TOTAL          1,325,000                           $21,326,566   $5,331.64
                  =========                           ===========   =========
</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 Nasdaq
         National Market System as of a date January 3,2001.



EXPLANATORY NOTE

         This Registration Statement relates to the registration of 1,325,000
additional shares of Common Stock authorized for issuance under the Plan. 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 prospectuses filed as a part of
the Company's registration statements No. 33-41534 and No. 33-73544 may no
longer be used for reofferings and resales of Common stock because the
information in such prospectuses is no longer accurate.


                                       2


<PAGE>


                                     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, 2000.

         (b)      The Registrant's quarterly report on Form 10-Q (Commission
                  File No. 0-17999) for the quarter ended September 30, 2000.

         (c)      The Registrant's current reports on Form 8-K filed with the
                  Commission on September 11, 2000, October 10, 2000 and
                  November 16, 2000 and on Form 8-K/A filed with the Commission
                  on October 10, 2000.

         (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 (File No. 0-17999), filed with
                  the Commission on September 22, 1989, including any amendment
                  or report filed for the purpose of updating such description.

         (e)      The Registrant's proxy materials on Schedule 14A filed with
                  the Commission October 12, 2000.

         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-3 (Registration No. 33-48042), as amended.

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)


                                       3


<PAGE>


(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 PricewaterhouseCoopers

(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; (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;
         Notwithstanding the foregoing, any increase or decrease in volume of
         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 Securities 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; provided, however, that paragraphs
         (a)(1)(i) and (ii) shall not apply if the information required to be
         included in a post-effective amendment by those paragraphs is contained
         in periodic reports filed by the registrant pursuant to section 13 or
         section 15(d) of the Securities and Exchange Act of 1934 that are
         incorporated by reference in the 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 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


                                       4


<PAGE>


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.







                                       5


<PAGE>


                                   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 to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Cambridge, Commonwealth of Massachusetts, on January
4,2001.

                                            IMMUNOGEN, INC.


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

         We, the undersigned officers and directors of ImmunoGen, Inc., hereby
severally constitute and appoint Mitchel Sayare and Walter Blattler and each of
them singly, as true and lawful attorneys, with full power to them and each of
them singly, to sign for us in our names in the capacities indicated below, all
additional amendments to this registration statement, and generally to do all
things in our names and on our behalf in such capacities to enable ImmunoGen,
Inc. to comply with the provisions of the Securities Act of 1933, as amended,
and all applicable requirements of the Securities and Exchange Commission.

         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>
Title:                                                                 Date:
<S>                        <C>                                         <C>
/s/ Mitchel Sayare         Chairman of the Board                       January 4, 2001
---------------------      of Directors and
  Mitchel Sayare           Chief Executive Officer
                           (principal executive officer)

/s/ Walter A. Blattler
---------------------      Executive Vice President,                   January 4, 2001
  Walter A. Blattler       Science and Technology,
                           Treasurer and Director


/s/ Virginia A. Lavery     Senior Corporate Controller                 January 4, 2001
---------------------      (principal accounting officer)
  Virginia A. Lavery


/s/ David W. Carter        Director                                    January 4, 2001
---------------------
  David W. Carter


/s/ Michael R. Eisenson    Director                                    January 4, 2001
---------------------
  Michael R. Eisenson


/s/ Stuart F. Feiner       Director                                    January 4, 2001
---------------------
  Stuart F. Feiner


/s/ Mark S. Skaletsky      Director                                    January 4, 2001
---------------------
  Mark S. Skaletsky
</TABLE>


                                       6


<PAGE>


                                 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 PricewaterhouseCoopers

(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)



                                       7
[{"type": "S-8", "sequence": "1", "filename": "a2034549zs-8.txt", "description": "S-8", "secsgml_size_bytes": 19317}]
{"company-data": {"conformed-name": "IMMUNOGEN INC", "cik": "0000855654", "assigned-sic": "2834", "irs-number": "042726691", "state-of-incorporation": "MA", "fiscal-year-end": "0630"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-53292", "film-number": "1502822"}, "business-address": {"street1": "128 SIDNEY ST", "city": "CAMBRIDGE", "state": "MA", "zip": "02139", "phone": "6176619312"}, "mail-address": {"street1": "128 SIDENEY ST", "city": "CAMBRIDGE", "state": "MA", "zip": "02139"}}
As filed with the Securities and Exchange Commission
                               On January 5, 2001

                              Registration No. 33-

                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                    FORM S-8
                             REGISTRATION STATEMENT

                                      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.)

                128 SIDNEY STREET, CAMBRIDGE, MASSACHUSETTS 02139
                -------------------------------------------------
                    (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.
                                128 Sidney Street
                         Cambridge, Massachusetts 02139
                         ------------------------------
                     (NAME AND ADDRESS OF AGENT FOR SERVICE)

                                 (617) 995-2500
                                 --------------
          (TELEPHONE NUMBER, INCLUDING AREA CODE, OF AGENT FOR SERVICE)

                  Please send copies of all communications to:

                            Jonathan L. Kravetz, Esq.
                           Mintz, Levin, Cohn, Ferris,
                             Glovsky and Popeo, P.C.
                              One Financial Center
                                Boston, MA 02111
                                 (617) 542-6000

                                       1
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,
$.01 par value      472,163 (2)         $ 8.14        $ 3,843,407   $  960.85

Common Stock,
$.01 par value      852,837 (3)          20.50         17,483,158   $4,370.79
                  ---------

   TOTAL          1,325,000                           $21,326,566   $5,331.64
                  =========                           ===========   =========
(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 Nasdaq
         National Market System as of a date January 3,2001.

EXPLANATORY NOTE

         This Registration Statement relates to the registration of 1,325,000
additional shares of Common Stock authorized for issuance under the Plan. 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 prospectuses filed as a part of
the Company's registration statements No. 33-41534 and No. 33-73544 may no
longer be used for reofferings and resales of Common stock because the
information in such prospectuses is no longer accurate.

                                       2
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, 2000.

         (b)      The Registrant's quarterly report on Form 10-Q (Commission
                  File No. 0-17999) for the quarter ended September 30, 2000.

         (c)      The Registrant's current reports on Form 8-K filed with the
                  Commission on September 11, 2000, October 10, 2000 and
                  November 16, 2000 and on Form 8-K/A filed with the Commission
                  on October 10, 2000.

         (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 (File No. 0-17999), filed with
                  the Commission on September 22, 1989, including any amendment
                  or report filed for the purpose of updating such description.

         (e)      The Registrant's proxy materials on Schedule 14A filed with
                  the Commission October 12, 2000.

         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-3 (Registration No. 33-48042), as amended.

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)

                                       3
(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 PricewaterhouseCoopers

(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; (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;
         Notwithstanding the foregoing, any increase or decrease in volume of
         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 Securities 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; provided, however, that paragraphs
         (a)(1)(i) and (ii) shall not apply if the information required to be
         included in a post-effective amendment by those paragraphs is contained
         in periodic reports filed by the registrant pursuant to section 13 or
         section 15(d) of the Securities and Exchange Act of 1934 that are
         incorporated by reference in the 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 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

                                       4
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.

                                       5
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 to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Cambridge, Commonwealth of Massachusetts, on January
4,2001.

                                            IMMUNOGEN, INC.

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

         We, the undersigned officers and directors of ImmunoGen, Inc., hereby
severally constitute and appoint Mitchel Sayare and Walter Blattler and each of
them singly, as true and lawful attorneys, with full power to them and each of
them singly, to sign for us in our names in the capacities indicated below, all
additional amendments to this registration statement, and generally to do all
things in our names and on our behalf in such capacities to enable ImmunoGen,
Inc. to comply with the provisions of the Securities Act of 1933, as amended,
and all applicable requirements of the Securities and Exchange Commission.

         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.
Title:                                                                 Date:
/s/ Mitchel Sayare         Chairman of the Board                       January 4, 2001
---------------------      of Directors and
  Mitchel Sayare           Chief Executive Officer
                           (principal executive officer)

/s/ Walter A. Blattler
---------------------      Executive Vice President,                   January 4, 2001
  Walter A. Blattler       Science and Technology,
                           Treasurer and Director

/s/ Virginia A. Lavery     Senior Corporate Controller                 January 4, 2001
---------------------      (principal accounting officer)
  Virginia A. Lavery

/s/ David W. Carter        Director                                    January 4, 2001
---------------------
  David W. Carter

/s/ Michael R. Eisenson    Director                                    January 4, 2001
---------------------
  Michael R. Eisenson

/s/ Stuart F. Feiner       Director                                    January 4, 2001
---------------------
  Stuart F. Feiner

/s/ Mark S. Skaletsky      Director                                    January 4, 2001
---------------------
  Mark S. Skaletsky
6
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 PricewaterhouseCoopers

(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)

                                       7