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                              CIBA-GEIGY AG, etal.

                                 [LETTERHEAD]


                               LETTER AGREEMENT

                                                        Basle, May 6, 1996

Dear Sirs:

We refer to the agreements set forth on Annex 1 hereto, among one or more of
us and you (collectively, the "Agreements"). Capitalized terms used in this
Letter Agreement but not defined shall have the meanings set forth in the
Agreements.


Ciba-Geigy-Limited ("Ciba") and Sandoz Limited ("Sandoz") have agreed to
effect a business combination, subject to approval by the relevant
governmental entities. To effectuate this business combination, Ciba and
Sandoz will form Novartis Limited, a Swiss corporation ("Novartis"), that
will be owned initially 50% by Ciba and 50% by Sandoz. Ciba and Sandoz will
thereafter combine with Novartis, as a result of which all the assets and
liabilities of Ciba and Sandoz will become assets and liabilities of
Novartis, each of Ciba and Sandoz will be absorbed by Novartis and cease
separate legal existence, and the stockholders of Ciba and Sandoz will become
the stockholders of Novartis (such transactions being referred to as the
"Combination").




In connection with the Combination, we ask you to agree as follows:

1. If and to the extent applicable to any of the Agreements, including
   Section 5.01 of the Governance Agreement, and subject to execution and
   delivery of the

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   assignment and assumption agreements referred to in the next
   paragraph, Chiron (i) consents to the ownership by Novartis or any wholly
   owned subsidiary of Novartis of shares of Common Stock or shares of any Ciba
   subsidiary holding shares of Common Stock, in each case as and to the extent
   that the Agreements would, without regard to this Letter Agreement, permit
   Ciba to hold such shares of Common Stock or shares of a Ciba subsidiary
   holding such shares of Common Stock, (ii) agrees to waive any violations of
   any Agreement that may result solely from such ownership, and (iii) agrees
   that the Agreements shall be deemed amended to permit such ownership.



2. In connection with the Combination, Ciba's rights and obligations under
   the Agreements shall be assigned to, and assumed by, Novartis, and rights and
   obligations under the Agreements of any subsidiary of Ciba may be assigned
   to, and assumed by, a subsidiary of Novartis, by operation of law. Ciba
   agrees that Chiron may request in writing that Novartis deliver within a
   reasonable period following any such assignment and assumption an executed
   written assumption agreement reasonably satisfactory to Chiron pursuant to
   which Novartis or the applicable subsidiary of Novartis, agrees to be bound
   by the provisions of such Agreement. Subject to compliance with the preceding
   sentence, Chiron consents to such assignment and assumption of each
   Agreement, to the extent applicable to each such Agreement. Upon each such
   assignment and assumption, all references to Ciba or subsidiaries of Ciba in
   the Agreement assigned and assumed shall be deemed references to Novartis or
   subsidiaries of Novartis, as the case may be, all references to Ciba-Geigy
   Corporation or Ciba Biotech Partnership, Inc. shall be deemed references to
   the wholly owned subsidiary of Novartis assuming the obligations of such
   entity, and such Agreement shall be deemed to be amended
   accordingly.


3. Chiron confirms that, after giving effect to this Letter Agreement, the
   consummation of the Combination will not conflict with or give rise to any
   right of termination, cancellation or acceleration by Chiron or any of its
   subsidiaries, or the loss by Ciba or any of its subsidiaries of any benefit
   under, any of the Agreements.




4. The provisions of this Letter Agreement that amend, modify or otherwise
   affect any Agreement shall be governed by and construed in accordance with
   the choice of law provisions of such Agreement. All other provisions of this
   Letter Agreement shall be governed by and construed in accordance with the
   laws of the State of Delaware, regardless of the laws that might otherwise
   govern under applicable principles of conflicts of law. As used herein, the
   term "including" means including, without limitation. Except as expressly set
   forth herein, following the Combination each Agreement shall remain in full
   force and effect in accordance with its terms.

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                                    - 3 -

If the foregoing is in accordance with your understanding of our agreement,
please sign and return to us, whereupon this letter and your acceptance shall
represent a binding agreement among each of us.


                                               Very truly yours,
                                               Ciba-Geigy Limited


                             by:

                             Name: /s/ Peter Sidler  /s/ Dr. Herbert Gut
                                  ------------------ -------------------------
                             Title: Peter Sidler     Dr. Herbert Gut
                                    Senior Tax and   Senior Division Counsel
                                   Corporate Counsel

                                         Ciba-Geigy Corporation


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




                                         Ciba Biotech Partnership, Inc.


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


The foregoing is hereby confirmed and accepted as of the date first above
written.




                                         CHIRON Corporation


                             by:
                             Name: /s/ William G. Green
                                  -----------------------
                             Title: Senior Vice President


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                                                                        ANNEX A
                                                            to Letter Agreement



The Investment Agreement dated as of November 20, 1994 among Ciba-Geigy
Limited, a Swiss Corporation ("Ciba"), Ciba-Geigy Corporation, a New York
Corporation ("CGC"), Ciba Biotech Partnership, Inc., a Delaware Corporation
("Biotech"), and Chiron Corporation, a Delaware Corporation ("Chiron"), the
Governance Agreement dated as of November 20, 1994 (the "Governance Agreement")
among Ciba, CGC and Chiron, Market Price Option Agreement dated as of
November 20, 1994 among Ciba, CGC, Biotech and Chiron, Subscription Agreement
dated as of November 20, 1994 among Ciba, Biotech and Chiron, Cooperation and
Collaboration Agreement dated as of November 20, 1994 between Ciba and
Chiron, the Registration Rights Agreement between Biotech and Chiron dated as
of November 20, 1994, the IgF Termination Agreement dated as of January 7,
1994 between Ciba and Chiron, the Registration and License Agreement on
Optical Mapping and Sequencing Technology (including the Side Letter thereto
to which Ciba is a party) dated as of March 31, 1995 between Chiron and New
York University, the CRU Research Services and Toll Research Services
Agreements dated as of January 4, 1995 between Ciba Corning Diagnostics Corp.
and Ciba, the Reimbursement Agreement dated as of March 24, 1995 between Ciba
and Chiron, the NYU Registration and License Agreement dated as of April 2,
1995 between Ciba and Chiron and the Technology Transfer and Research
Collaboration Agreement dated as of November 15, 1995 between Ciba and Chiron.