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                                                  EXHIBIT 10.27


THIS DOCUMENT CONSTITUTES PART OF THE OFFICIAL PROSPECTUS COVERING
SECURITIES THAT HAVE BEEN REGISTERED UNDER THE SECURITIES ACT OF
1933

                               December 12, 1991

Dear ______________________

Re: AMENDMENT OF OPTION AGREEMENT UNDER THE CETUS CORPORATION AMENDED AND
RESTATED COMMON STOCK OPTION PLAN

     Our records indicate that you held an option under the above-referenced
plan ("Plan"). On December 12, 1991, pursuant to the provisions of the Chiron
1991 Stock Option Plan described in the proxy/prospectus dated October 30,
1991 and pursuant to the merger ("Merger") described in an Agreement and Plan
of Merger ("Agreement") dated as of July 21, 1991 by and among Chiron
Corporation - ("Chiron"), Cetus Corporation ("Cetus") and Chiron Acquisition
Subsidiary, Inc., Chiron assumed all unexpired, unexercised options to
purchase shares of Cetus Common Stock ("Cetus Options") and all related
unexpired, unexercised stock appreciation rights under the Plan.

     Pursuant to this assumption, effective as of the effective date of the
Merger, each Cetus Option Agreement is amended as follows:

     1.    Each Cetus Option is converted into an option ("Chiron Option") to
purchase shares of Chiron Common Stock.

     2.    The number of shares purchasable under, and the exercise price of,
options under the Plan are adjusted as follows to take into account the
difference in value between Cetus Common Stock and Chiron Common Stock:

           (a) The number of shares of Chiron Common Stock (rounded to the
nearest whole number) purchasable under a Chiron Option equals the number of
shares of Cetus Common Stock that could have been purchased under the
replaced Cetus Option multiplied by the Exchange Ratio in the Merger, which
is .3.

           (b) The per share exercise price of each Chiron Option equals the
per share exercise price determined pursuant to the replaced Cetus Option
divided by the Exchange Ratio in the Merger, which is .3.

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Page 2.
December 12, 1991


     3.    The terms and conditions of the Chiron Options are administered
and interpreted by Chiron's Board of Directors or by a Committee appointed by
Chiron's Board of Directors with references to Cetus deemed references to
Chiron where appropriate.

     4.    For purposes of vesting, service as an Advisory Counsellor (as
defined in the Agreement) constitutes service as a director.

     5.    If Cetus Options for which Chiron Options are substituted have
Limited Stock Appreciation Rights ("LSARS") attached thereto, then each such
LSAR will be honored by Chiron in accordance with its terms and remain
exercisable for a period of 60 days following the date that stockholders of
Cetus approve the Merger; provided, however, that if the LSAR was originally
granted within 6 months of the date that Cetus stockholders approve the
Merger, then the LSAR will be exercisable for a period of 60 days following
expiration of such 6-month period. Upon expiration of the applicable 60 day
period, each such LSAR not previously exercised shall expire. Upon exercise
of an LSAR, the related option will be cancelled, and Chiron will pay to the
LSAR holder an amount in cash for each share with respect to which the LSAR
is exercised determined in accordance with the terms of the Cetus Corporation
Amended and Restated Common Stock Option Plan. This provision shall not
affect obligations under any other agreement regarding whether or not the
holder of an LSAR will exercise that LSAR.

     Except as provided above, the grant date, the expiration date and the
vesting schedule of each Chiron Option will remain the same as the grant
date, the expiration date and the vesting schedule of the Cetus Option that
it replaces. Chiron and Cetus intend that each Chiron Option will constitute
an incentive stock option within the meaning of Section 422 of the Internal
Revenue Code to the extent that the Cetus Option it replaces would have
constituted an incentive stock option prior to December 12, 1991.

Please keep this amendment with the documentation that was forwarded to you
before the Merger under the Cetus Corporation Amended and Restated Common
Stock Option Plan. If you have any questions about this amendment, please
contact ___________________________________.

Very truly yours,




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THIS DOCUMENT CONSTITUTES PART OF THE OFFICIAL PROSPECTUS COVERING
SECURITIES THAT HAVE BEEN REGISTERED UNDER THE SECURITIES ACT OF
1933

                               December 12, 1991

Dear______________________

Re: AMENDMENT OF OPTION AGREEMENT UNDER THE CETUS CORPORATION NON-EMPLOYEE
DIRECTORS' STOCK OPTION PLAN

     Our records indicate that you held an option under the above-referenced
plan ("Plan"). On December 12, 1991, pursuant to the provisions of the Chiron
1991 Stock Option Plan described in the proxy/prospectus dated October 30,
1991 and pursuant to the merger ("Merger") described in an Agreement and Plan
of Merger ("Agreement") dated as of July 21, 1991 by and among Chiron
Corporation - ("Chiron"), Cetus Corporation ("Cetus") and Chiron Acquisition
Subsidiary, Inc., Chiron assumed all unexpired, unexercised options to
purchase shares of Cetus Common Stock ("Cetus Options") and all related
unexpired, unexercised stock appreciation rights under the Plan.

     Pursuant to this assumption, effective as of the effective date of the
Merger, each Cetus Option Agreement is amended as follows:

     1.    Each Cetus Option is converted into an option ("Chiron Option") to
purchase shares of Chiron Common Stock.

     2.    The number of shares purchasable under, and the exercise price of,
options under the Plan are adjusted as follows to take into account the
difference in value between Cetus Common Stock and Chiron Common Stock:

           (a) The number of shares of Chiron Common Stock (rounded to the
nearest whole number) purchasable under a Chiron Option equals the number of
shares of Cetus Common Stock that could have been purchased under the
replaced Cetus Option multiplied by the Exchange Ratio in the Merger, which
is .3.

           (b) The per share exercise price of each Chiron Option equals the
per share exercise price determined pursuant to the replaced Cetus Option
divided by the Exchange Ratio in the Merger, which is .3.

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Page 2.
December 12, 1991

     3.    The terms and conditions of the Chiron Options are administered
and interpreted by Chiron's Board of Directors or by a Committee appointed by
Chiron's Board of Directors with references to Cetus deemed references to
Chiron where appropriate.

     4.    For purposes of vesting, service as an Advisory Counsellor (as
defined in the Agreement) constitutes service as a director.

     5.    If Cetus Options for which Chiron Options are substituted have
Limited Stock Appreciation Rights ("LSARS") attached thereto, then each such
LSAR will be honored by Chiron in accordance with its terms and remain
exercisable for a period of 60 days following the date that stockholders of
Cetus approve the Merger; provided, however, that if the LSAR was originally
granted within 6 months of the date that Cetus stockholders approve the
Merger, then the LSAR will be exercisable for a period of 60 days following
expiration of such 6-month period. Upon expiration of the applicable 60 day
period, each such LSAR not previously exercised shall expire. Upon exercise
of an LSAR, the related option will be cancelled, and Chiron will pay to the
LSAR holder an amount in cash for each share with respect to which the LSAR
is exercised determined in accordance with the terms of the Cetus Corporation
Non-Employee Directors' Stock Option Plan. This provision shall not affect
obligations under any other agreement regarding whether or not the holder of
an LSAR will exercise that LSAR.

     Except as provided above, the grant date, the expiration date and the
vesting schedule of each Chiron Option will remain the same as the grant
date, the expiration date and the vesting schedule of the Cetus Option that
it replaces.

     Please keep this amendment with the documentation that was forwarded to
you before the Merger under the Cetus Corporation Non-Employee Directors'
Stock Option Plan. If you have any questions about this amendment, please
contact ____________________.

Very truly yours,