<PAGE>

               AMENDMENT WITH RESPECT TO EMPLOYEE STOCK OPTION
          ARRANGEMENTS, dated January 3, 1995, among CIBA-GEIGY
          LIMITED, a Swiss corporation ("Ciba"), CIBA-GEIGY
          CORPORATION, a New York corporation ("CCorp"), CIBA BIOTECH
          PARTNERSHIP, INC., a Delaware corporation ("Holdings"), and
          CHIRON CORPORATION, a Delaware corporation (the "Company").

     WHEREAS Ciba, CCorp, Holdings and the Company have entered into the
Investment Agreement dated as of November 20, 1994 (the "Investment
Agreement") (capitalized terms used but not defined herein shall have the
meanings assigned to them in the Investment Agreement);

     WHEREAS the Investment Agreement provides in Section 5.14 thereof for the
implementation of arrangements regarding Employee Stock Options;

     WHEREAS the parties wish to make certain changes to the arrangements
provided in Section 5.14 as set forth below;

     NOW, THEREFORE, in consideration of the transactions contemplated by the
Investment Agreement and for other good and valuable consideration, the
sufficiency and receipt of which are hereby acknowledged, the parties hereto
hereby agree as follows:

     SECTION 1. The parties hereto agree that, notwithstanding anything to
the contrary contained in Section 5.14 of the Investment Agreement:

     (a) All payments to be made to holders of Stock Options pursuant to
  Section 5.14 shall be made by Ciba direct to the person entitled thereto, and
  not by the Company. The parties shall work out methods and procedures for
  effecting such payments, and the Company will provide, at its own expense,
  such information and administrative support services as shall be required
  therefor, including information regarding required withholdings. Such payments
  shall be made, and the related arrangements shall be effected, at no added
  expense to Ciba. Information detailing each amount so paid and each amount
  withheld with respect thereto and the nature thereof shall be prepared by the
  Company for the benefit of both parties. The parties shall cooperate with
  respect to the application of and arrangements related to such withheld
  amounts.

     (b) The definition of "Options Tranche" set forth in the last sentence of
   Section 5.14(a) shall be amended to read in full as follows: "An 'Options
   Tranche' shall mean that portion of a series of options

<PAGE>

  with the same expiration date and exercise price that vest on the same date."

         (c) The parties intend, and the last proviso of Section 5.14(b)
  shall be interpreted to mean, that only such portion of a Type-A Non-Vested
  Options with respect to which the Type-A Non-Vested Option Payment Right was
  exercised shall be cancelled.

         (d) Pursuant to Section 5.14(d) the Company has determined that
  shareholder approval is desirable with respect to any right to a payment under
  Section 5.14 that may be made to any director or officer as defined in Rule
  16a-1 of the Exchange Act ("Section 16(b) Persons"). Accordingly, the
  parties have agreed that the right of any Section 16(b) Person to payment
  under Section 5.14 shall be embodied in an amendment to the Company's Stock
  Option plans, which amendment shall be submitted to the stockholders of the
  Company for approval at the next regularly scheduled meeting. Any right to
  such payment is expressly conditioned upon such approval being obtained, and
  no such payment shall be made prior thereto. Upon obtaining such approval,
  such payment shall be made by Ciba to each Section 16(b) Person in an amount
  equal to the aggregate Option Participation Payment such person would have
  been entitled to under Section 5.14 had such payment been made on the Closing
  Date, increased at a rate of 6% per annum calculated from the Closing Date to
  the date such payment shall actually be made by Ciba; provided that such
  increased payments shall be required only for amounts payable under Sections
  5.14(a) and (b) for Vested Option Payment Rights and Type A Non-Vested
  Option Payment Rights, as the case may be, exercised on or prior to the fifth
  business day after such approval is obtained and for all amounts payable
  under Section 5.14(c).

         (e) The parties agree that Ciba shall have the right to review at
  all reasonable times the books, records and other information of the Company
  relating to the derivation and recipients of amounts paid by Ciba pursuant to
  Section 5.14, as amended, to verify that such amounts paid were correctly
  paid and calculated.

         (f) The parties agree that Ciba shall have the right at any time to
  review the books, records and any other information of the Company relating
  to the derivation and recipients of amounts paid by Ciba pursuant hereto to
  verify that such amounts paid were correctly paid and calculated.

                                      2.

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         (g) The Company agrees to indemnify and hold harmless Ciba and its
  subsidiaries and affiliates and any of their present, former and future
  directors, officers, employees, trustees, representatives, attorneys and
  advisers from, against and in respect of any damages, claims, losses, charges,
  actions, suits, proceedings, deficiencies, taxes, interest, penalties and
  reasonable costs and expenses (including attorneys' fees) ("Liabilities")
  imposed on, sustained, incurred or suffered by or asserted against any of
  them, directly or indirectly, relating to, arising out of or in connection
  with, Ciba's payments to holders of Stock Options in accordance herewith,
  excluding any Liabilities that would have been imposed on Ciba relating to,
  arising out of or in connection with Section 5.14 as the same shall have been
  in effect on the date of the execution of the Investment Agreement and without
  giving effect to this Amendment, in any case including, but not limited to,
  the reimbursement by the Company of any amounts paid by Ciba that were not
  properly payable in accordance with Section 5.14 of the Investment Agreement
  and the terms hereof.

         (h) The parties hereby acknowledge and agree that the Section
  references in the proviso to Section 5.14(e) are to "Section 5.14(a)" and
  "Section 5.14(b)", respectively.

         SECTION 2. This Amendment constitutes the entire agreement and
  understanding of the parties with respect to the subject matter hereof and
  supersedes any and all prior agreements and understandings, oral or written,
  relating to the subject matter hereof. Capitalized terms used herein and not
  otherwise defined herein shall have the meaning assigned to such terms in the
  Investment Agreement. This Supplemental Agreement shall be construed in
  accordance with and governed by the law of the State of Delaware. This
  Supplemental Agreement may be executed in any number of counterparts, each of
  which shall be an original but all of which, when taken together, shall
  constitute but one instrument. Except as expressly set forth herein, this
  Supplemental Agreement shall not by implication or otherwise limit, impair,
  constitute a waiver of, or otherwise affect the rights and remedies of the
  parties under the Investment Agreement or any of the Ancillary Agreements.

                                      3.

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                  IN WITNESS WHEREOF, the parties hereto have caused this
         Amendment with Respect Stock Option Arrangements to be duly executed by
         their respective authorized officers as of the date first above
         written.

                                  CIBA-GEIGY LIMITED,

                                       by      /s/           Dr. Herbert Gut
                                           -------------------------------------
                                           Name:            Dr. Herbert Gut
                                           Title:        Senior Division Counsel

                                       by      /s/              Peter Sidler
                                           -------------------------------------
                                           Name:       Peter Sidler
                                           Title:     Senior Tax and
                                                        Corporate Counsel

                                  CIBA-GEIGY CORPORATION,

                                       by
                                           -------------------------------------
                                           Name:
                                           Title:


                                  CIBA BIOTECH PARTNERSHIP, INC.

                                       by
                                           -------------------------------------
                                           Name:
                                           Title:


                                  CHIRON CORPORATION

                                       by
                                           -------------------------------------
                                           Name:
                                           Title:

<PAGE>

                  IN WITNESS WHEREOF, the parties hereto have caused this
         Amendment with Respect Stock Option Arrangements to be duly executed by
         their respective authorized officers as of the date first above
         written.

                                  CIBA-GEIGY LIMITED,

                                       by
                                           -------------------------------------
                                           Name:
                                           Title:

                                       by
                                           -------------------------------------
                                           Name:
                                           Title:


                                  CIBA-GEIGY CORPORATION,

                                       by      /s/  John J. McGraw
                                           -------------------------------------
                                           Name:
                                           Title:


                                  CIBA BIOTECH PARTNERSHIP, INC.

                                       by      /s/  John J. McGraw
                                           -------------------------------------
                                           Name:
                                           Title:


                                  CHIRON CORPORATION

                                       by
                                           -------------------------------------
                                           Name:
                                           Title:

<PAGE>

                  IN WITNESS WHEREOF, the parties hereto have caused this
         Amendment with Respect Stock Option Arrangements to be duly executed by
         their respective authorized officers as of the date first above
         written.

                                  CIBA-GEIGY LIMITED,

                                       by
                                           -------------------------------------
                                           Name:
                                           Title:

                                       by
                                           -------------------------------------
                                           Name:
                                           Title:


                                  CIBA-GEIGY CORPORATION,

                                       by
                                           -------------------------------------
                                           Name:
                                           Title:


                                  CIBA BIOTECH PARTNERSHIP, INC.

                                       by
                                           -------------------------------------
                                           Name:
                                           Title:


                                  CHIRON CORPORATION

                                       by    /s/    William J. Rutter
                                           -------------------------------------
                                           Name:
                                           Title: