Exhibit 10.515
Special Executive Form

CHIRON CORPORATION.                            , 2002

Dear
Stock Option Plan Participant:

I
am pleased to inform you that the Compensation Committee of the Board of Directors recently approved an amendment to each of your outstanding stock options listed on Exhibit A
(the "Option") to provide you with greater protection in the event of certain corporate transactions.

The
amendment is set forth in detail below. In general terms, the amendment does the following:

First,
the amendment provides that your Option will immediately vest upon a Qualifying Termination of your employment that occurs within twenty-four (24) months after
a Change in Control. (See below for definitions.)

Second,
the amendment provides that your Option will vest immediately before certain corporate transactions if your Option will not continue or be assumed by the successor corporation or
replaced with a comparable option. Those transactions generally include a sale or disposition of all or substantially all of the assets of the Corporation, a merger or consolidation with another
entity or a reorganization or liquidation of the Corporation. Before the amendment, this protection was not available for a stock acquisition or reorganization involving less than substantially all of
the Corporation's capital stock.

Accordingly,
Section C of your stock option agreement applicable to your Option is hereby amended in its entirety to read as follows:

C.    Change In Control.

1.Failure to Assume.If the Corporation or its stockholders enter into an agreement to dispose of all or
substantially all of the assets of the Corporation, enter into an agreement to merge or consolidate with another entity or enter into a plan of reorganization or liquidation, then this Option, to the
extent not previously exercised or terminated, may be exercised, immediately before the consummation of such transaction with respect to all the shares of Common Stock purchasable under this Option,
unless this Option will be either continued or assumed by the successor entity or its parent or replaced with a comparable option to purchase equity interests of the successor entity or parent
thereof. The Committee will determine such comparability, and its determination will be final, binding and conclusive. Upon consummation of such a transaction, this Option, whether or not accelerated,
will terminate and cease to be exercisable, unless continued or assumed by the successor entity or parent thereof.

2.Certain Terminations Following a Change in Control.If there is a Change in Control of the Corporation pursuant to which the
exercisability of the Option does not accelerate in full pursuant to 1. above and within twenty-four (24) calendar months thereafter there is a Qualifying Termination of Optionee's
employment, then this Option will become immediately exercisable for all of the Option Shares then subject to this Option. For this purpose, the following definitions apply:



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Notwithstanding
anything else contained herein to the contrary, in no event shall a Change in Control be deemed to have occurred by reason of a purchase, or series of purchases of Corporation stock by
Novartis or its successor such that the acquiring entity remains subject to the terms of that certain Governance Agreement dated as of January 5, 1995, as amended through December 9,
2000, provided the acquiring entity's Corporation stock holdings, direct or indirect, in the aggregate, represent less than seventy-nine and nine-tenths of a percent (79.9%) of
the combined voting power of all outstanding Corporation securities. In addition, in no event shall a Change in Control be deemed to have occurred, with respect to the Optionee, if the Optionee is
part of a purchasing group that consummates the Change-in-Control transaction. The Optionee shall be deemed "part of a purchasing group" for purposes of the preceding sentence
if the Optionee is an equity participant in the purchasing company or group (except for: (i) passive ownership of less than three percent (3%) of the stock or other equity of the purchasing
company; or (ii) ownership of equity participation in the purchasing company or group which is otherwise not significant, as determined prior to the Change in Control by a majority of the
nonemployee continuing Directors).



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For
purposes of this subparagraph, no act, or failure to act, on the Optionee's part shall be deemed "willful" unless done, or omitted to be done, by the Optionee not in good faith and without
reasonable belief that the action or omission was in the best interests of the Corporation.



However,
the occurrence of an event set forth in (i) through (iv) above shall not constitute Good Reason if the Corporation has cured such event within fifteen (15) days of
receipt of written notice from the Optionee that such event has occurred and constitutes Good Reason.

3.Right to Effect Transactions.This Agreement does not in any way affect the right of the Corporation to adjust, reclassify, reorganize
or otherwise change its capital or business structure or to merge, consolidate, dissolve, liquidate or sell or transfer all or any part of its business or assets.

Except
for the foregoing change to Section C of the option agreement applicable to your Option, no other terms or conditions of your Option or the underlying option shares have
been modified as a result of the amendment, and those other terms and conditions will continue in full force and effect.

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Please attach a copy of this letter agreement to your stock option agreement so that you will have a complete record of all the terms applicable to your Option.

The
amendment was effective as of February 16, 2001. We hope that you find this amendment a valuable addition to your equity package. Should you have any questions concerning the
amendment, please direct them to the undersigned.

| |  | Very truly yours, 
| |  | ------------------
| |  |                   
| |  | Chiron Corporation
| |  |                   
| |  |                   
| |  | By:               
| |  | Title:            

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EXHIBIT A

|Name |  | Date of Grant |  | Exercise Price
Per Share |  | Total Number of
Option Shares Granted |  | Total Number of
Option Shares
Outstanding
|---- |  | ------------- |  | ------------------------ |  | ------------------------------------- |  | -----------------------------------------
|     |  |               |  |                          |  |                                       |  |                                          

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