<PAGE>


        As filed with the Securities and Exchange Commission on June 2, 1997
                                                 Registration No. 33-___________

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

                                       FORM S-8
                               REGISTRATION STATEMENT
                                        UNDER
                              THE SECURITIES ACT OF 1933
                                  ------------------
                                  CHIRON CORPORATION
                (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)

         Delaware                                        94-2754624
(State or other jurisdiction of                       (I.R.S. Employer
incorporation or organization)                        Identification No.)
                    4560 Horton St., Emeryville, California 94608
                                    (510) 655-8730
            (Address, including zip code, and telephone number, including
               area code, of registrant's principal executive offices)

                                  ------------------

                            CHIRON 1991 STOCK OPTION PLAN
                               (Full title of the plan)

                                  ------------------

                                WILLIAM G. GREEN, ESQ.
                      SENIOR VICE PRESIDENT AND GENERAL COUNSEL
                                  CHIRON CORPORATION
                   4560 Horton Street, Emeryville, California 94608
                                    (510) 655-8730
              (Name, address, including zip code, and telephone number,
                      including area code, of agent for service)

                           CALCULATION OF REGISTRATION FEE
<TABLE>
<CAPTION>
------------------------------------------------------------------------------------------------------------------------------------
Title of Securities                                       Proposed Maximum               Proposed Maximum             Amount of
to be Registered        Amount to be Registered       Offering Price Per Share      Aggregate Offering Price      Registration Fee
-------------------     -----------------------       ------------------------      ------------------------      ----------------
<S>                     <C>                           <C>                           <C>                           <C>
Common Stock, $0.01     5,000,000 (1)                 $18.8125 (2)                   $94,062,500 (2)                    $28,504
par  value (1991 Stock
Option Plan)
------------------------------------------------------------------------------------------------------------------------------------
</TABLE>

(1) This Registration Statement shall also cover any additional shares of
    Common Stock which become issuable under the Chiron 1991 Stock Option Plan,
    by reason of any stock dividend, stock split, recapitalization or other
    similar transaction effected without the receipt of consideration which
    results in an increase in the number of the Registrant's outstanding shares
    of Common Stock.
(2) Calculated solely for the purposes of this offering under Rule 457(h) of
    the Securities Act of 1933 on the basis of the average of the high and low
    selling prices per share of Common Stock of Chiron Corporation on May 28,
    1997, as reported by the NASDAQ National Market System.

This Registration Statement shall become effective immediately upon filing with
the Securities and Exchange Commission, and sales of the registered securities
will begin as soon as reasonably practicable after such effective date.
                                  ------------------

Also relates to Registration File Nos. 33-35182, 33-20181, 2-90595, 33-44477,
33-65024, 33-65177 and 333-10419 pursuant to Rule 429 under the Securities Act
of 1933.


<PAGE>

                                       PART II

                  INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

    The contents of the Registration Statement on Form S-8 (File No. 33-44477),
filed by Chiron Corporation (the "Registrant") with the Securities and Exchange
Commission on December 12, 1991, are hereby incorporated by reference herein.

ITEM 5.  INTERESTS OF NAMED EXPERTS AND COUNSEL.

    William G. Green, who has provided an opinion to the Registrant on the 
validity of the securities being registered which is Exhibit 5 to this 
registration statement, is Senior Vice President, General Counsel and 
Secretary of the Registrant.

ITEM 8.  EXHIBITS.

Exhibit Number     Exhibit
--------------     -------


    5              Opinion of William G. Green.

    23.1           Consent of KPMG Peat Marwick LLP, Independent Auditors.

    23.2           Consent of William G. Green is contained in Exhibit 5.

    24             Power of Attorney.  Reference is made to pages II-2 and II-3
                   of this Registration Statement.

    99.1           Chiron 1991 Stock Option Plan.

    99.2           Form of Automatic Share Right Agreement.

    99.3           Form of Performance Unit Agreement.


                                         II-1


<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 Emeryville, State of California, on the 30th day of
May, 1997.


                                  CHIRON CORPORATION

                                  By /s/ Edward E. Penhoet
                                     ----------------------------------------
                                     Edward E. Penhoet, Ph.D.
                                     President and Chief Executive Officer


                                  POWER OF ATTORNEY

    KNOW ALL PERSONS BY THESE PRESENTS, that the undersigned officers and
directors of Chiron Corporation, a Delaware corporation, do hereby constitute
and appoint Edward E. Penhoet, Ph.D. and William J. Rutter, Ph.D., and each of
them, the lawful attorneys and agents or attorney and agent with full power and
authority to do any and all acts and things and to execute any and all
instruments which said attorneys and agents, and any one of them, determine may
be necessary or advisable or required to enable said corporation to comply with
the Securities Act of 1933, as amended, and any rules or regulations or
requirements of the Securities and Exchange Commission in connection with this
Registration Statement.  Without limiting the generality of the foregoing power
and authority, the powers granted include the power and authority to sign the
names of the undersigned officers and directors in the capacities indicated
below this Registration Statement, to any and all amendments, both pre-effective
and post-effective, and supplements to this Registration Statement, and to any
and all instruments or documents filed as part of or in conjunction with this
Registration Statement or amendments or supplements thereof, and each of the
undersigned hereby ratifies and confirms all that said attorneys and agents, or
any one of them shall do or cause to be done by virtue hereof.  This Power of
Attorney may be signed in several counterparts.

    IN WITNESS WHEREOF, each of the undersigned has executed this Power of
Attorney as of the date indicated.


                                         II-2


<PAGE>

    Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed below by the following persons in the
capacities and on the date indicated.

SIGNATURE                       TITLE                             DATE

/s/ Edward E. Penhoet, Ph.D.    President, Chief Executive        May 30, 1997
------------------------------  Officer and Director (Principal
(Edward E. Penhoet, Ph.D.)      Executive Officer)

/s/ William J. Rutter, Ph.D.    Chairman of the Board             May 30, 1997
------------------------------
(William J. Rutter, Ph.D.)

/s/ Dennis L. Winger            Senior Vice President, Finance    May 30, 1997
------------------------------  and Administration, Chief
(Dennis L. Winger)              Financial Officer (Principal
                                Financial and Accounting
                                Officer)

/s/ Lewis W. Coleman            Director                          May 30, 1997
------------------------------
(Lewis W. Coleman)

/s/ Pierre E. Douaze            Director                          May 30, 1997
------------------------------
(Pierre E. Douaze)

/s/ Donald A. Glaser, Ph.D.     Director                          May 30, 1997
------------------------------
(Donald A. Glaser, Ph.D.)

/s/ Alex Krauer, Ph.D.          Director                          May 30, 1997
------------------------------
(Alex Krauer, Ph.D.)

/s/ Henri Schramek, Ph.D.       Director                          May 30, 1997
------------------------------
(Henri Schramek, Ph.D.)

/s/ Jack W. Schuler             Director                          May 30, 1997
------------------------------
(Jack W. Schuler)

/s/ Pieter J. Strijkert, Ph.D.  Director                          May 30, 1997
------------------------------
(Pieter J. Strijkert, Ph.D.)

/s/ Paul L. Herrling            Director                          May 30, 1997
------------------------------
(Paul L. Herrling, Ph.D.)


                                         II-3


<PAGE>

                          SECURITIES AND EXCHANGE COMMISSION
                               WASHINGTON, D.C.  20549


                                       EXHIBITS
                                          TO
                                       FORM S-8
                                      UNDER THE
                                SECURITIES ACT OF 1933

                                  CHIRON CORPORATION


<PAGE>

                                    EXHIBIT INDEX



Exhibit Number     Exhibit
--------------     -------

    5              Opinion of William G. Green.

    23.1           Consent of KPMG Peat Marwick LLP, Independent Auditors.

    23.2           Consent of William G. Green is contained in Exhibit 5.

    24             Power of Attorney.  Reference is made to pages II-2 and II-3
                   of this Registration Statement.

    99.1           Chiron 1991 Stock Option Plan.

    99.2           Form of Automatic Share Right Agreement.

    99.3           Form of Performance Unit Agreement.
[{"type": "S-8", "sequence": "1", "description": "FORM S-8", "secsgml_size_bytes": 10980}]
{"company-data": {"conformed-name": "CHIRON CORP", "cik": "0000706539", "assigned-sic": "2834", "irs-number": "942754624", "state-of-incorporation": "DE", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-28257", "film-number": "97617929"}, "business-address": {"street1": "4560 HORTON ST", "city": "EMERYVILLE", "state": "CA", "zip": "94608", "phone": "5106558730"}}
As filed with the Securities and Exchange Commission on June 2, 1997
                                                 Registration No. 33-___________

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

                                       FORM S-8
                               REGISTRATION STATEMENT
                                        UNDER
                              THE SECURITIES ACT OF 1933
                                  ------------------
                                  CHIRON CORPORATION
                (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)

         Delaware                                        94-2754624
(State or other jurisdiction of                       (I.R.S. Employer
incorporation or organization)                        Identification No.)
                    4560 Horton St., Emeryville, California 94608
                                    (510) 655-8730
            (Address, including zip code, and telephone number, including
               area code, of registrant's principal executive offices)

                                  ------------------

                            CHIRON 1991 STOCK OPTION PLAN
                               (Full title of the plan)

                                  ------------------

                                WILLIAM G. GREEN, ESQ.
                      SENIOR VICE PRESIDENT AND GENERAL COUNSEL
                                  CHIRON CORPORATION
                   4560 Horton Street, Emeryville, California 94608
                                    (510) 655-8730
              (Name, address, including zip code, and telephone number,
                      including area code, of agent for service)

                           CALCULATION OF REGISTRATION FEE
------------------------------------------------------------------------------------------------------------------------------------
Title of Securities                                       Proposed Maximum               Proposed Maximum             Amount of
to be Registered        Amount to be Registered       Offering Price Per Share      Aggregate Offering Price      Registration Fee
-------------------     -----------------------       ------------------------      ------------------------      ----------------
Common Stock, $0.01     5,000,000 (1)                 $18.8125 (2)                   $94,062,500 (2)                    $28,504
par  value (1991 Stock
Option Plan)
------------------------------------------------------------------------------------------------------------------------------------
(1) This Registration Statement shall also cover any additional shares of
    Common Stock which become issuable under the Chiron 1991 Stock Option Plan,
    by reason of any stock dividend, stock split, recapitalization or other
    similar transaction effected without the receipt of consideration which
    results in an increase in the number of the Registrant's outstanding shares
    of Common Stock.
(2) Calculated solely for the purposes of this offering under Rule 457(h) of
    the Securities Act of 1933 on the basis of the average of the high and low
    selling prices per share of Common Stock of Chiron Corporation on May 28,
    1997, as reported by the NASDAQ National Market System.

This Registration Statement shall become effective immediately upon filing with
the Securities and Exchange Commission, and sales of the registered securities
will begin as soon as reasonably practicable after such effective date.
                                  ------------------

Also relates to Registration File Nos. 33-35182, 33-20181, 2-90595, 33-44477,
33-65024, 33-65177 and 333-10419 pursuant to Rule 429 under the Securities Act
of 1933.
PART II

                  INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

    The contents of the Registration Statement on Form S-8 (File No. 33-44477),
filed by Chiron Corporation (the "Registrant") with the Securities and Exchange
Commission on December 12, 1991, are hereby incorporated by reference herein.

ITEM 5.  INTERESTS OF NAMED EXPERTS AND COUNSEL.

    William G. Green, who has provided an opinion to the Registrant on the 
validity of the securities being registered which is Exhibit 5 to this 
registration statement, is Senior Vice President, General Counsel and 
Secretary of the Registrant.

ITEM 8.  EXHIBITS.

Exhibit Number     Exhibit
--------------     -------

    5              Opinion of William G. Green.

    23.1           Consent of KPMG Peat Marwick LLP, Independent Auditors.

    23.2           Consent of William G. Green is contained in Exhibit 5.

    24             Power of Attorney.  Reference is made to pages II-2 and II-3
                   of this Registration Statement.

    99.1           Chiron 1991 Stock Option Plan.

    99.2           Form of Automatic Share Right Agreement.

    99.3           Form of Performance Unit Agreement.

                                         II-1
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 Emeryville, State of California, on the 30th day of
May, 1997.

                                  CHIRON CORPORATION

                                  By /s/ Edward E. Penhoet
                                     ----------------------------------------
                                     Edward E. Penhoet, Ph.D.
                                     President and Chief Executive Officer

                                  POWER OF ATTORNEY

    KNOW ALL PERSONS BY THESE PRESENTS, that the undersigned officers and
directors of Chiron Corporation, a Delaware corporation, do hereby constitute
and appoint Edward E. Penhoet, Ph.D. and William J. Rutter, Ph.D., and each of
them, the lawful attorneys and agents or attorney and agent with full power and
authority to do any and all acts and things and to execute any and all
instruments which said attorneys and agents, and any one of them, determine may
be necessary or advisable or required to enable said corporation to comply with
the Securities Act of 1933, as amended, and any rules or regulations or
requirements of the Securities and Exchange Commission in connection with this
Registration Statement.  Without limiting the generality of the foregoing power
and authority, the powers granted include the power and authority to sign the
names of the undersigned officers and directors in the capacities indicated
below this Registration Statement, to any and all amendments, both pre-effective
and post-effective, and supplements to this Registration Statement, and to any
and all instruments or documents filed as part of or in conjunction with this
Registration Statement or amendments or supplements thereof, and each of the
undersigned hereby ratifies and confirms all that said attorneys and agents, or
any one of them shall do or cause to be done by virtue hereof.  This Power of
Attorney may be signed in several counterparts.

    IN WITNESS WHEREOF, each of the undersigned has executed this Power of
Attorney as of the date indicated.

                                         II-2
Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed below by the following persons in the
capacities and on the date indicated.

SIGNATURE                       TITLE                             DATE

/s/ Edward E. Penhoet, Ph.D.    President, Chief Executive        May 30, 1997
------------------------------  Officer and Director (Principal
(Edward E. Penhoet, Ph.D.)      Executive Officer)

/s/ William J. Rutter, Ph.D.    Chairman of the Board             May 30, 1997
------------------------------
(William J. Rutter, Ph.D.)

/s/ Dennis L. Winger            Senior Vice President, Finance    May 30, 1997
------------------------------  and Administration, Chief
(Dennis L. Winger)              Financial Officer (Principal
                                Financial and Accounting
                                Officer)

/s/ Lewis W. Coleman            Director                          May 30, 1997
------------------------------
(Lewis W. Coleman)

/s/ Pierre E. Douaze            Director                          May 30, 1997
------------------------------
(Pierre E. Douaze)

/s/ Donald A. Glaser, Ph.D.     Director                          May 30, 1997
------------------------------
(Donald A. Glaser, Ph.D.)

/s/ Alex Krauer, Ph.D.          Director                          May 30, 1997
------------------------------
(Alex Krauer, Ph.D.)

/s/ Henri Schramek, Ph.D.       Director                          May 30, 1997
------------------------------
(Henri Schramek, Ph.D.)

/s/ Jack W. Schuler             Director                          May 30, 1997
------------------------------
(Jack W. Schuler)

/s/ Pieter J. Strijkert, Ph.D.  Director                          May 30, 1997
------------------------------
(Pieter J. Strijkert, Ph.D.)

/s/ Paul L. Herrling            Director                          May 30, 1997
------------------------------
(Paul L. Herrling, Ph.D.)

                                         II-3
SECURITIES AND EXCHANGE COMMISSION
                               WASHINGTON, D.C.  20549

                                       EXHIBITS
                                          TO
                                       FORM S-8
                                      UNDER THE
                                SECURITIES ACT OF 1933

                                  CHIRON CORPORATION
EXHIBIT INDEX

Exhibit Number     Exhibit
--------------     -------

    5              Opinion of William G. Green.

    23.1           Consent of KPMG Peat Marwick LLP, Independent Auditors.

    23.2           Consent of William G. Green is contained in Exhibit 5.

    24             Power of Attorney.  Reference is made to pages II-2 and II-3
                   of this Registration Statement.

    99.1           Chiron 1991 Stock Option Plan.

    99.2           Form of Automatic Share Right Agreement.

    99.3           Form of Performance Unit Agreement.