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                                                                Exhibit 10.502

                                    EXHIBIT A

                               CHIRON CORPORATION

                       2000-1 FORM STOCK OPTION AGREEMENT

     A. MANNER OF EXERCISING OPTION.

          1.  To  the  extent  exercisable  under  the  Grant  Notice  and  this
Agreement,  this Option may be exercised before the Expiration Date set forth in
the Grant Notice (or earlier  termination of this Option) by delivering  written
notice  in such  form  and to such  person  as the  Corporation  specifies.  The
exercise date will be the date that this notice is delivered  ("Exercise Date").
The Option holder must furnish such documentation as the Corporation may request
to verify that such person has the right to exercise this Option.  The holder of
this Option  will not have any  stockholder  rights  with  respect to the Option
Shares until such person has exercised the Option,  paid the Exercise  Price and
become a holder of record of the purchased shares.

          2. Upon exercise, the Optionee must pay the full Exercise Price to the
Corporation in one or more of the following forms:  cash, shares of Common Stock
of the Corporation held by the Option holder for the requisite period to avoid a
charge to the  Corporation's  earnings  and valued as of the Exercise  Date,  or
delivery in a manner approved by the Corporation of irrevocable  instructions to
a broker to  promptly  deliver  to the  Corporation  the  amount of sale or loan
proceeds to pay the Exercise  Price.  In order to assist in the  acquisition  of
shares of Common Stock pursuant to this Agreement, the Committee may (but is not
required to) authorize the extension of a loan to Optionee from the Corporation,
the payment by Optionee of the Exercise Price in installments, or a guarantee by
the Corporation of a loan obtained by Optionee from a third party.

          3. For purposes of this Option, Fair Market Value of a share of Common
Stock on any date will be determined under the terms of the Plan, which, as long
as  the  Common  Stock  is  traded  on  the  NASDAQ  system  and  the  Committee
administering the Plan does not determine otherwise,  will be the average of the
high and low price of one share of Common Stock during NASDAQ's  regular trading
session, as determined by the Committee using such publicly available sources as
it may specify.

     B.  TERMINATION.   If  the  Optionee's  employment  terminates  before  the
Expiration  Date,  this Option will expire early and cease to be  exercisable in
accordance with the following provisions.  For purposes of these provisions, the
term "employment" includes service as an employee, an independent  contractor or
a consultant for the Corporation or a parent or subsidiary entity.

               a. If  Optionee  terminates  employment  other  than by reason of
Retirement,   death,  a  disability   that  qualifies  for  benefits  under  the
Corporation's  long-term  disability  program,  or Cause (each as defined below)
during the  option  term,  this  Option  will  terminate  on the  earlier of the
Expiration  Date or 3 months after the date of termination  of  employment.  For
purposes of this  paragraph  B, an employee  who goes on an approved  disability
leave will be deemed to have terminated  employment on the earlier of the end of
such leave (if Optionee has not returned to employment  with the  Corporation on
or before the end of such leave) or one year after the beginning of the leave.

               b. If Optionee  terminates  employment by  Retirement  during the
option term, this Option will terminate on the earlier of the Expiration Date or
36 months after such termination of employment.  Retirement means termination of
employment by an employee (by reason other than death or Cause) after all of the
following  has  occurred  (i) the  Optionee  has  completed  5 or more  years of
continuous  employment,  (ii) the Optionee has attained age 55 and (iii) the sum
of the Optionee's age in full years and full years of employment equals at least
65.

               c. If Optionee  takes an  approved  disability  leave  during the
option  term and  qualifies  for  benefits  under  the  Corporation's  long-term
disability insurance program (but not for Retirement), the Option will terminate
on the earlier of the Expiration Date or 36 months after the start of the leave

               d. If Optionee is on an approved  leave of absence  without  pay,
each Vesting Date (i.e.,  each date on which  Optionee  would  otherwise  become
eligible to exercise this option for the initial or additional  number of shares
if he or she is then still employed by the Corporation) shall be deferred by the
number of days of such  unpaid  leave (in excess of 31 in the case of a personal
leave),  provided  that in the  case  of a leave  during  which  Optionee  has a
statutory or contractual right to reemployment for some period, vesting shall be
deferred  by the number of days by which such leave  extends  beyond the date of
lapse of such right.

               e. If Optionee dies while this Option is outstanding,  the person
or entity who succeeds to this Option will have the following period to exercise
this  Option:  (i) if Optionee is  employed  by the  Corporation  at the time of
death,  the 36 month  period  following  the  date of  death,  (ii) if  Optionee
terminated  employment  before death by reason other than  Retirement,  Cause or
disability  that  qualifies  for  benefits  under  the  Corporation's  long-term
disability program,  the 12 month period following the date of death or (iii) if
Optionee  terminated   employment  before  death  by  reason  of  Retirement  or
disability  that  qualifies  for  benefits  under  the  Corporation's  long term
disability  program, 36 months after the date of such Retirement or, in the case
of a qualifying  disability,  the start of the disability leave.  However, in no
event can this Option be exercised after the Expiration Date.

               f. After Optionee's termination of employment, this Option cannot
be exercised for more than that number of Option Shares (if any) for which it is
exercisable  on the date of such  termination,  provided  that,  if  termination
occurs by  reason of death or  Retirement  (including  termination  by reason of
disability  AFTER becoming  eligible for  Retirement),  then during the 36 month
period of limited  exercisability  following such  termination of employment the
Option will continue to vest and become (and



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remain)  exercisable  as if the Optionee  continued  as an employee  during such
period.  Upon the expiration of any applicable limited period of exercise or (if
earlier) upon the  Expiration  Date,  this Option will terminate and cease to be
outstanding.

               g.  Notwithstanding  any  provision  of  this  Agreement  to  the
contrary,  if Optionee's  employment  is terminated  for Cause or if, during any
limited period of exercisability  following  termination of employment by reason
of Retirement or disability that qualifies for benefits under the  Corporation's
long-term  disability  program,  Optionee  engages in any  activity  which would
justify  termination for Cause,  then this Option will terminate and cease to be
outstanding  on the date of such  termination of employment or, if such activity
occurs after  termination of employment,  the date the Optionee  engages in such
activity. For purposes of this Agreement,  Cause includes, but is not be limited
to,  (i) any act of  dishonesty,  willful  misconduct,  fraud  or  embezzlement,
committed while an employee or which is against the  Corporation,  its parent or
one of its subsidiaries (the "Companies") or (ii) any unauthorized disclosure of
confidential  information  or trade secrets of any of the Companies  (whether or
not in violation of any  confidentiality  agreement).  If any  provision of this
paragraph contravenes or is invalid under the applicable laws of any state, this
entire  paragraph  will not be  invalidated,  but will be construed and enforced
insofar as the laws of that state are concerned as not containing the particular
provision or provisions that is invalid in that state.

     C. CHANGE IN CONTROL.  If the Corporation or its stockholders enter into an
agreement to dispose of all or  substantially  all of the assets or  outstanding
capital stock of the  Corporation by means of sale,  merger,  reorganization  or
liquidation  then  this  Option,  to the  extent  not  previously  exercised  or
terminated, may be exercised,  immediately before the consummation of such sale,
merger,  reorganization  or liquidation with respect to all the shares of Common
Stock purchasable under this Option. However, no such acceleration will occur if
the agreement  requires that each  outstanding  option will be either assumed by
the successor  corporation or its parent or replaced with a comparable option to
purchase shares of capital stock of the successor corporation or parent thereof.
The Committee will determine such  comparability,  and its determination will be
final,   binding  and  conclusive.   Upon  consummation  of  the  sale,  merger,
reorganization  or  liquidation  contemplated  by the  agreement,  this  Option,
whether or not accelerated,  will terminate and cease to be exercisable,  unless
assumed pursuant to a written  agreement by the successor  corporation or parent
thereof.  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.

     D. ADJUSTMENT IN OPTION SHARES. If there is a stock split,  stock dividend,
recapitalization,  combination  of shares,  exchange  of shares or other  change
affecting  the  outstanding  Common Stock as a class  without the  Corporation's
receipt of consideration,  appropriate adjustments will be made to (i) the total
number and/or class of  securities  subject to this Option and (ii) the Exercise
Price in order to reflect  such  change  and  thereby  preclude  a  dilution  or
enlargement of benefits hereunder.

     E. LIMITED  TRANSFERABILITY.  This Option cannot be transferred or assigned
other  than by will or by the  laws of  descent  and  distribution  and,  during
Optionee's lifetime, can only be exercised by Optionee, unless and to the extent
that this Option is originally designated a Non-Statutory Option, in which case,
subject to such  limitations  as the Company may establish from time to time for
administrative   and  regulatory   compliance   reasons   (including   continued
eligibility of Registration  Form-S-8),  this Option may be assigned by Optionee
by gift or pursuant to a domestic  relations  order to one or more of Optionee's
family  members or an entity owned,  benefiting or controlled by the Optionee or
one or more of Optionee's  family members.  Notwithstanding  the foregoing,  the
Optionee may also designate a beneficiary to whom this Option will automatically
be transferred if still outstanding upon the Optionee's death. The terms of this
Option are binding upon the executors, administrators, successors and assigns of
the Optionee.

     F. COMPLIANCE WITH PLAN, LAWS AND REGULATIONS.  The exercise of this Option
and the issuance of the Option Shares upon such exercise is subject to the terms
of the Plan and compliance by the  Corporation  and Optionee with all applicable
laws and with all  applicable  regulations  of any stock  exchange  on which the
Common  Stock is listed for trading at the  relevant  time.  If the  Corporation
cannot obtain approval from any regulatory body having  authority  deemed by the
Corporation  to be necessary to the lawful  issuance of Common Stock pursuant to
this Option,  the  Corporation  will not have any liability  with respect to the
failure to issue the Common Stock as to which such approval is not obtained.

     G. ADDITIONAL  TERMS APPLICABLE TO AN INCENTIVE  OPTION.  If this Option is
designated  an Incentive  Option in the Grant Notice,  the  following  terms and
conditions will also apply to the grant:

               a. This Option will not qualify for favorable tax treatment as an
Incentive  Option to the  extent  that it is  exercised:  (A) more than 3 months
after the date Optionee ceases to be an employee for any reason other than death
or  permanent  and total  disability  or (B) more than 12 months  after the date
Optionee  ceases to be an employee by reason of permanent and total  disability.
For this  purpose (i)  permanent  and total  disability  means the  inability to
engage  in  any  substantial   gainful  activity  by  reason  of  any  medically
determinable  physical or mental  impairment  which can be expected to result in
death or which has lasted or can be expected to last for a continuous  period of
not less than 12  months  and (ii) an  employee  on a leave of  absence  will be
deemed to have  terminated  employment  on the 91st day after  the  employee  no
longer has a contractual or statutory right to reemployment.

               b. To the extent  that the  aggregate  Fair  Market  Value of the
stock  for  which  options  intended  to  be  Incentive   Options  first  become
exercisable  during any calendar  year (under this Plan or any other plan of the
Corporation  or any  parent or  subsidiary  and  whether  by  reason of  initial
installment  exercisability or acceleration of  exercisability  upon a Change in
Control) would exceed  $100,000 in the aggregate,  the options shall not qualify
as Incentive  Options,  but shall be exercisable as Non-Statutory  Options.  The
foregoing  limit will be applied by taking into account  options in the order in
which they were granted.

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