<PAGE>

                                                                 EXECUTION COPY

                        SUPPLEMENTAL AGREEMENT dated as of 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 for Ciba and CCorp to
cause Holdings to transfer to the Company the Diagnostics Shares and the Ciba
Biocine Business as consideration for 6,600,000 newly issued shares of Common
Stock to be purchased by Holdings;

     WHEREAS the parties wish to structure the transfer of the
Contributed Businesses in the forms described herein;

     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 the Investment Agreement, at the Closing the
transfer of the Contributed Businesses to the Company shall be effected as
follows:

     (a) Ciba and CCorp shall cause JV US Holdings to merge with and into
  Chiron Biocine Acquisition Corporation and the Company shall cause Chiron
  Biocine Acquisition Corporation to merge with JV US Holdings, with Chiron
  Biocine Acquisition Corporation as the surviving corporation, all upon the
  terms and subject to the conditions of an agreement and plan of merger
  substantially in the form of Exhibit A hereto. The consideration to be
  received in the merger by the holders of shares of common stock of JV US
  Holdings shall include the amounts owed by the Company to Ciba

<PAGE>

                                                                              2

  or its subsidiaries pursuant to Section 1.04(iv)(3) of the Investment
  Agreement and consummation of such merger shall satisfy the Company's
  obligation to repay such amounts at the Closing pursuant to such Section 1.04
  (iv)(3).

     (b) The Company shall cause Chiron CCD Acquisition Corporation
  to merge with and into Diagnostics and Ciba and CCorp shall cause
  Diagnostics to merge with Chiron CCD Acquisition Corporation, with
  Diagnostics as the surviving corporation, all upon the terms and
  subject to the conditions of an agreement and plan of merger
  substantially in the form of Exhibit B hereto.

     (c) Upon the terms and subject to the conditions of the
  Investment Agreement, the Company shall purchase from Ciba, CCorp or a
  subsidiary of Ciba, and Ciba, CCorp or a subsidiary of Ciba shall sell,
  transfer and deliver to the Company, the JV Vax Shares, in consideration
  for which the Company shall issue, sell, transfer and deliver to Ciba, CCorp,
  Holdings, or a subsidiary of Ciba 700,000 New Shares.

     SECTION 2. The parties hereto hereby agree that the transfer of the
Contributed Businesses pursuant to SECTION 1 hereof rather than according to
ARTICLE I of the Investment Agreement and the satisfaction of the Company's
obligation under Section 1.04(iv)(3) of the Investment Agreement by means
of consummation of the merger contemplated by SECTION 1(a) hereof shall be
deemed not to result in (i) any representation or warranty set forth in the
Investment Agreement or any Ancillary Agreement being false or incorrect in
any respect and (ii) any covenant or agreement set forth in the Investment
Agreement or any Ancillary Agreement being breached in any respect.

     SECTION 3. This Supplemental Agreement 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. 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

<PAGE>

                                                                              3

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.

<PAGE>

                  IN WITNESS WHEREOF, the parties hereto have caused this
         Supplemental Agreement to be duly executed by their respective
         authorized officers as of the date first above written.

                                  CIBA-GEIGY LIMITED,

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

                                       by      /s/ Steven Ballmer
                                           -------------------------------------
                                           Name:   Steven Ballmer
                                           Title:  Tax 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
         Supplemental Agreement 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:  John J. McGraw
                                           Title: Vice President and
                                                    General Counsel

                                  CIBA BIOTECH PARTNERSHIP,
                                  INC.,

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


                                  CHIRON CORPORATION,

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

<PAGE>

                                                                      EXHIBIT A

                        AGREEMENT AND PLAN OF MERGER dated
                   as of January 4, 1995 (hereinafter called
                   this "AGREEMENT"), among CIBA-GEIGY BIOCINE
                   CORPORATION, a Delaware corporation
                   ("BIOCINE"), CHIRON CORPORATION, a Delaware
                   corporation ("CHIRON") and CHIRON BIOCINE
                   ACQUISITION CORPORATION, a Delaware
                   corporation and a wholly owned subsidiary of
                   Chiron ("B ACQUISITION").


     WHEREAS, Biocine desires to merge with and into B Acquisition and B
Acquisition desires to merge with Biocine, all upon the terms and subject to
the conditions of this Agreement; and

     WHEREAS, the respective Boards of Directors of Biocine and B Acquisition
have approved the merger of Biocine into B Acquisition as set forth below
(the "MERGER"), upon the terms and conditions set forth in this Agreement.

     NOW, THEREFORE, in consideration of the mutual promises and conditions
contained herein, the parties hereto hereby agree as follows:

     SECTION 1. THE MERGER. Upon the terms and subject to the conditions of
this Agreement and in accordance with the Delaware General Corporation Law
(the "DGCL"), at the Effective Time (as hereinafter defined) Biocine shall be
merged with and into B Acquisition (as of the Effective Time, the "SURVIVING
CORPORATION") and the separate corporate existence of Biocine shall thereupon
cease. Following the Merger, the Surviving Corporation shall succeed to and
assume all the rights and obligations of Biocine in accordance with the DGCL,
and the separate corporate existence of the Surviving Corporation as a
corporation under the laws of the State of Delaware, with all its rights,
privileges, powers, immunities, purposes and franchises, shall continue
unaffected by the Merger. The Merger shall have the effects set forth in the
DGCL, including Section 259 thereof.

     SECTION 2. CONDITIONS OF MERGER; EFFECTIVE TIME. After this Agreement
has been approved and adopted by the affirmative vote or consent of the
stockholders of each of Biocine and B Acquisition, in each case in accordance
with

<PAGE>

                                                                              2

their respective certificates of incorporation and the DGCL, the parties
hereto shall cause a certificate of merger or other appropriate documents
meeting the requirements of the DGCL (in any such case, the "CERTIFICATE OF
MERGER") to be properly executed in accordance with the relevant provisions
of the DGCL and filed in accordance with the DGCL. The Merger shall become
effective as of the date and time when the Delaware Certificate of Merger is
duly filed with the Secretary of State of the State of Delaware (the date and
time the Merger becomes effective being the "EFFECTIVE TIME").

     SECTION 3. CERTIFICATE OF INCORPORATION; BY-LAWS; DIRECTORS; OFFICERS.

     (a) CERTIFICATE OF INCORPORATION. The Certificate of Incorporation of B
Acquisition, as in effect immediately prior to the Effective Time, shall
continue in full force and effect and shall be the Certificate of
Incorporation of the Surviving Corporation, until thereafter changed or
amended as provided therein or by applicable law.

     (b) BY-LAWS. As of the Effective Time, the By-laws of B Acquisition, as
in effect immediately prior to the Effective Time, shall continue in full
force and effect and shall be the By-laws of the Surviving Corporation, until
thereafter changed or amended as provided therein or by applicable law.

     (c) DIRECTORS. The directors of B Acquisition at the Effective Time
shall be the directors of the Surviving Corporation, until the earlier of
their resignation or removal or until their respective successors are duly
elected and qualified, as the case may be.

     (d) OFFICERS. The officers of B Acquisition at the Effective Time shall
be the officers of the Surviving Corporation, until the earlier of their
resignation or removal or until their respective successors are duly elected
and qualified, as the case may be.

     SECTION 4. CONVERSION OF SHARES. As of the Effective Time, and without
any further action on the part of any holder of the common stock, par value
$1.00 per share, of Biocine ("BIOCINE COMMON STOCK") or any holder of the
common stock, par value $0.01 per share, of B Acquisition ("B ACQUISITION
COMMON STOCK"), each of the

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                                                                              3

following transactions shall be deemed to occur simultaneously:

     (a) Each issued and outstanding share of Biocine Common Stock shall be
converted into and exchanged for (i) 3,900 fully paid and nonassessable
shares of the common stock, par value $0.01 per share, of Chiron ("CHIRON
COMMON STOCK") plus (ii) a cash payment in the amount of $[23,746.00].

     (b) Each share of B Acquisition Common Stock issued and outstanding
immediately prior to the Effective Time shall remain issued and outstanding
and shall constitute, after the Effective Time, one fully paid and
nonassessable share of the common stock, par value $0.01 per share, of the
Surviving Corporation.

     SECTION 5. EXCHANGE OF CERTIFICATES. If a holder of record (immediately
prior to the Effective Time) of shares of Biocine Common Stock outstanding
immediately prior to the Effective Time presents (after the Effective Time)
to Chiron a certificate or certificates which immediately prior to the
Effective Time represented such shares, then, in exchange for the surrender
and cancelation of such certificate or certificates, Chiron shall issue to
such holder of record a certificate or certificates representing the shares
of Chiron Common Stock and pay to such holder of record the amount of cash
into which such shares of Biocine Common Stock were converted in accordance
with Section 4. Until surrendered as contemplated by this Section 5, each
certificate which immediately prior to the Effective Time represented
outstanding shares of Biocine Common Stock shall be deemed to represent only
the right to receive upon such surrender a certificate representing the
shares of Chiron Common Stock and the amount of cash into which such shares
of Biocine Common Stock were converted, as contemplated by this Section 5.

     SECTION 6. MODIFICATION OR AMENDMENT. This Agreement may be amended or
modified, at any time prior to the filing of the Certificate of Merger with
the Secretary of State of the State of Delaware, by the mutual written
consents of Biocine and B Acquisition, by action of their respective Boards
of Directors; PROVIDED, HOWEVER, that after any required approval of the
Merger by the stockholders of Biocine or B Acquisition, there shall be made
no amendment that by law requires further approval by

<PAGE>

                                                                              4

such stockholders without the further approval of such stockholders.

     SECTION 7. COUNTERPARTS. This Agreement may be executed in one or more
counterparts, all of which shall be considered one and the same agreement and
shall become effective when one or more counterparts have been signed by each
of the parties and delivered to the other parties.

     SECTION 8. GOVERNING LAW. This Agreement shall be governed by and
construed in accordance with the laws of the State of Delaware.

<PAGE>

                                                                              5

     IN WITNESS WHEREOF, this Agreement has been duly executed and
  delivered by the duly authorized officers of the parties hereto on the
  date first hereinabove written.


                                  CIBA-GEIGY BIOCINE
                                  CORPORATION,

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


                                  CHIRON CORPORATION,

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



                                  CHIRON BIOCINE ACQUISITION
                                  CORPORATION,

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

<PAGE>

                                                                      EXHIBIT B

                        AGREEMENT AND PLAN OF MERGER dated
                   as of January 4, 1995 (hereinafter called
                   this "AGREEMENT"), among CIBA CORNING
                   DIAGNOSTICS CORP., a Delaware corporation
                   ("CCD"), CHIRON CORPORATION, a Delaware
                   corporation ("CHIRON") and CHIRON CCD
                   ACQUISITION CORPORATION, a Delaware
                   corporation and a wholly owned subsidiary of
                   Chiron ("C ACQUISITION").


     WHEREAS, C Acquisition desires to merge with and into CCD and CCD
desires to merge with C Acquisition, all upon the terms and subject to the
conditions of this Agreement; and

     WHEREAS, the respective Boards of Directors of C Acquisition and CCD
have approved the merger of C Acquisition into CCD as set forth below (the
"MERGER"), upon the terms and conditions set forth in this Agreement.

     NOW, THEREFORE, in consideration of the mutual promises and conditions
contained herein, the parties hereto hereby agree as follows:

     SECTION 1. THE MERGER. Upon the terms and subject to the conditions of
this Agreement and in accordance with the Delaware General Corporation Law
(the "DGCL"), at the Effective Time (as hereinafter defined) C Acquisition
shall be merged with and into CCD (as of the Effective Time, the "SURVIVING
CORPORATION") and the separate corporate existence of C Acquisition shall
thereupon cease. Following the Merger, the Surviving Corporation shall
succeed to and assume all the rights and obligations of C Acquisition in
accordance with the DGCL, and the separate corporate existence of the
Surviving Corporation as a corporation under the laws of the State of
Delaware, with all its rights, privileges, powers, immunities, purposes and
franchises, shall continue unaffected by the Merger. The Merger shall have
the effects set forth in the DGCL, including Section 259 thereof.

     SECTION 2. CONDITIONS OF MERGER; EFFECTIVE TIME. After this Agreement
has been approved and adopted by the affirmative vote or consent of the
stockholders of each of C Acquisition and CCD, in each case in accordance
with their respective certificates of incorporation and the DGCL, the parties
hereto shall cause a certificate of merger or other appropriate documents
meeting the requirements of the DGCL

<PAGE>

                                                                              2

(in any such case,. the "CERTIFICATE OF MERGER") to be properly executed in
accordance with the relevant provisions of the DGCL and filed in accordance
with the DGCL. The Merger shall become effective as of the date and time when
the Delaware Certificate of Merger is duly filed with the Secretary of State
of the State of Delaware (the date and time the Merger becomes effective
being the "EFFECTIVE TIME").

     SECTION 3. CERTIFICATE OF INCORPORATION; BY-LAWS; DIRECTORS; OFFICERS.

     (a) CERTIFICATE OF INCORPORATION. The Certificate of Incorporation of
CCD, as in effect immediately prior to the Effective Time, shall continue in
full force and effect and shall be the Certificate of Incorporation of the
Surviving Corporation, until thereafter changed or amended as provided
therein or by applicable law.

     (b) BY-LAWS. As of the Effective Time, the By-laws of CCD, as in effect
immediately prior to the Effective Time, shall continue in full force and
effect and shall be the By-laws of the Surviving Corporation, until
thereafter changed or amended as provided therein or by applicable law.

     (c) DIRECTORS. The directors of C Acquisition at the Effective Time
shall be the directors of the Surviving Corporation, until the earlier of
their resignation or removal or until their respective successors are duly
elected and qualified, as the case may be.

     (d) OFFICERS. The officers of CCD at the Effective Time shall be the
officers of the Surviving Corporation, until the earlier of their resignation
or removal or until their respective successors are duly elected and
qualified, as the case may be.

     SECTION 4. CONVERSION OF SHARES. As of the Effective Time, and without
any further action on the part of any holder of the common stock, par value
$0.01 per share, of C Acquisition ("C ACQUISITION COMMON STOCK") or any
holder of the common stock, par value $10.00 per share, of CCD ("CCD COMMON
STOCK"), each of the following transactions shall be deemed to occur
simultaneously:

     (a) Each issued and outstanding share of C Acquisition Common Stock
shall be converted into and exchanged for ten fully paid and nonassessable
shares of

<PAGE>

                                                                               3

Common Stock, par value $10 per share, of the Surviving Corporation.

     (b) Each share of CCD Common Stock issued and outstanding immediately
prior to the Effective Time shall be converted into and exchanged for 200
fully paid and nonassessable shares of the common stock, par value $0.01 per
share, of Chiron ("Chiron Common Stock").

     SECTION 5. EXCHANGE OF CERTIFICATES. (a) If a holder of record
(immediately prior to the Effective Time) of shares of CCD Common Stock
outstanding immediately prior to the Effective Time presents (after the
Effective Time) to Chiron a certificate or certificates which immediately
prior to the Effective Time represented such shares, then Chiron shall issue
to such holder of record, in exchange for the surrender and cancelation of
such certificate or certificates, a certificate or certificates representing
the shares of Chiron Common Stock into which such shares of CCD Common Stock
were converted in accordance with Section 4. Until surrendered as
contemplated by this Section 5(a), each certificate which immediately prior
to the Effective Time represented outstanding shares of CCD Common Stock
shall be deemed to represent only the right to receive upon such surrender a
certificate representing the shares of Chiron Common Stock into which such
shares of CCD Common Stock were converted, as contemplated by this Section
5(a).

     (b) If a holder of record (immediately prior to the Effective Time) of
shares of C Acquisition Common Stock outstanding immediately prior to the
Effective Time presents (after the Effective Time) to CCD a certificate or
certificates which immediately prior to the Effective Time represented such
shares, then CCD shall issue to such holder of record, in exchange for the
surrender and cancelation of such certificate or certificates, a certificate
or certificates representing the shares of CCD Common Stock into which such
shares of C Acquisition Common Stock were converted in accordance with
Section 4. Until surrendered as contemplated by this Section 5(b), each
certificate which immediately prior to the Effective Time represented
outstanding shares of C Acquisition Common Stock shall be deemed to represent
only the right to receive upon such surrender a certificate representing the
shares of CCD Common Stock into which such shares of C Acquisition Common
Stock were converted, as contemplated by this Section 5(b).

     SECTION 6. MODIFICATION OR AMENDMENT. This Agreement may be amended or
modified, at any time prior to the filing of the Certificate of Merger with the
Secretary

<PAGE>

                                                                              4

of State of the State of Delaware, by the mutual written consents of C
Acquisition and CCD, by action of their respective Boards of Directors;
PROVIDED, HOWEVER, that after any required approval of the Merger by the
stockholders of C Acquisition or CCD, there shall be made no amendment that by
law requires further approval by such stockholders without the further approval
of such stockholders.

     SECTION 7. COUNTERPARTS. This Agreement may be executed in one or more
counterparts, all of which shall be considered one and the same agreement and
shall become effective when one or more counterparts have been signed by each of
the parties and delivered to the other parties.

     SECTION 8. GOVERNING LAW. This Agreement shall be governed by and
construed in accordance with the laws of the State of Delaware.

<PAGE>

                                                                              5

     IN WITNESS WHEREOF, this Agreement has been duly executed and delivered
by the duly authorized officers of the parties hereto on the date first
hereinabove written.

                                  CIBA CORNING DIAGNOSTICS
                                  CORP.,

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

                                  CHIRON CORPORATION,

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

                                  CHIRON CCD ACQUISITION
                                  CORPORATION,

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