EXHIBIT 10.611
 
 
CHIRON
 
 
                                   March 18, 1998
Mr. Sean P. Lance
25A Warrington Crescent
London W91ED
England
 
Dear Mr. Lance:
 
          On behalf of the senior management and all members of the Chiron
community, I am very pleased to extend to you our offer to lead our Company, as
President and Chief Executive Officer.  Subject to your agreement and approval
by the Board of Directors, which we will promptly obtain, this letter sets forth
the terms of your employment by Chiron.  
 
     1.   POSITION, DUTIES AND RESPONSIBILITIES.
 
          (a)  You shall be the President and CEO of the Company and shall
in such capacity report directly to the Company's Board of Directors (the
"Board").  Your duties and responsibilities will be determined from time to time
by the Board, and will be consistent with your position as President and Chief
Executive Officer.
 
          (b)  You shall be appointed to the Board of Directors as a Management
Director as defined in Section 2.01 of the Governance Agreement between the
Company and Novartis Ltd. dated November 20, 1994 ("Governance Agreement"), and
serve on such Committees of the Board as elected or appointed by the Board.  
 
          (c)  You shall devote your full business time, ability and attention
to the business of the Company, and shall not engage in or perform duties for
any other person which interfere with the performance of your duties hereunder. 
It is expected and desirable for you to hold board of director positions on
outside civic organizations, and that reasonable time will be made available to
fulfill your duties in that regard as long as those activities do not interfere
with the performance of your duties hereunder.  Any outside commercial board of
director's positions will be subject to approval by the Board.
 
          (d)  During any period with respect to which you are receiving or have
received payments from the Company hereunder, including any period following a
voluntary termination by you of your employment for Good Reason (as defined
below) but not including any period following an involuntary termination of your
employment by the Company other than for Cause (as defined below), you will not
(i) own (except as a passive investor in a publicly owned company), manage or be
employed in any manner with any enterprise which is engaged primarily in a
business competitive with the Company (currently biotechnology, pharmaceuticals,
vaccines and diagnostics) or to be engaged in any way in the management or
direction of that portion of an enterprise that is directly competitive with the
Company, or (ii) solicit the employment of any employee of the Company, or (iii)
solicit any customer of the Company, or interfere in any other manner with any
existing business relationship between the Company and any customer or other
third party.
 
<PAGE>
 
          (e)  You agree to sign the Company's standard proprietary information
agreement for employees.
 
     2.   SALARY AND BONUS COMPENSATION.
 
          (a)  BASE SALARY.  As compensation for your services hereunder
(including your services as a member of the Board) you shall receive a base
salary of $700,000 per annum.  This base salary may be increased, but not
decreased, annually by the Board, consistent with your performance and the
Company's policy regarding increases in officer compensation established from
time to time by the Board.
 
          (b)  BONUS COMPENSATION.  You may, in addition, at the Board's
discretion, be awarded incentive compensation, in the form of a cash bonus for
each fiscal year during your employment, under the Company's 1995 Executive
Officer Variable Cash Compensation Plan, or its successor, for each given year,
based upon performance.  If the targeted level of performance is satisfied, the
cash bonus amount will be equal to one hundred percent (100%) of your base
salary for the year in which you satisfied the bonus criteria, with a maximum
bonus of two hundred percent (200%) of base salary if your performance
substantially exceeds the targeted level.  There is no guaranteed minimum level
of bonus compensation, and the maximum bonus compensation that can be achieved
will be equal to two hundred percent (200%) of your base salary.
 
     3.   BENEFITS.
 
          (a)  STANDARD BENEFITS.  You shall be eligible to participate in
standard employee benefit programs (including medical, dental, life and
disability insurance, which shall be effective as of and from the date of your
employment hereunder) as the Company shall maintain from time to time for the
benefit of employees.  Attached as Exhibit A is a copy of the Company's current
employee benefits for your information.  You may receive such other and
additional benefits as the Board may determine from time to time in its sole
discretion.
 
          (b)  VACATION.  You shall be entitled to four (4) weeks paid vacation
per annum under the Company's integrated paid time off program for executives,
and with such additional paid vacation time as the Board may reasonably
determine or is consistent with the company's vacation policy as it exists from
time to time.  No unused vacation shall accrue and no unused vacation will be
paid on termination.
 
     4.   EXPENSE REIMBURSEMENT.
 
          The Company shall reimburse you for certain expenses under the
standard executive benefits program of the Company.  At a minimum, this will
include the lease of a company car and tax and financial planning services. 
Additionally, the Company will reimburse you in accordance with the Company's
reimbursement policies in effect from time to time for all reasonable and
customary business expenses incurred during your employment, provided that you
furnish to the Company reasonably adequate records and documentary evidence of
such expense.
 
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<PAGE>
 
     5.   LONG-TERM INCENTIVE COMPENSATION.
 
          Upon commencement of your employment at the Company you shall receive
the following grants in accordance with the terms of the Company's 1991 Stock
Option Plan, as amended.  The 1991 Stock Option Plan, as amended, is set forth
in the Company's Proxy Statement for its 1997 Annual Meeting, a copy of which
has been provided to you.
 
          (a)  STOCK OPTION.  You shall be granted a Stock Option for 750,000
shares of the Company's common stock exercisable at fair market value on the
date of grant and having a term of ten years.  The Stock Option shall be an
incentive stock option under the federal tax laws to the maximum extent
permissible, and the balance of the option shall be a non-statutory option.  You
shall vest in the option shares (i) as to 125,000 of the option shares at the
one-year anniversary of the start date of your employment with the Company, (ii)
as to 475,000 of the option shares in forty-eight (48) successive equal monthly
installments upon your completion of each month of service with the Company
measured from the first anniversary of your start date with the Company and
(iii) as to the remaining 150,000 of the option shares on the fifth anniversary
of your start date provided that up to 75,000 of these 150,000 option shares can
vest as early as the end of three years based on attainment of agreed upon
performance goals to be reflected in the option grant.
 
          (b)  ANNUAL AWARDS.  You will be eligible to participate in the
Company's annual awards to executives of long-term incentive compensation,
presently awarded under the 1991 Stock Option Plan, in the form of stock
options, based upon performance as determined by the Board's Compensation
Committee.  Under the program as presently administered, the target value of the
annual award at the grant date would be 200% of your base salary.  These awards
are highly discretionary and are subject to review and adjustment to reflect
changes in the Company and competitive market conditions.
 
     6.   RELOCATION ASSISTANCE PACKAGE.
 
          (a)  HOME EQUITY LOAN.  A loan shall be made available to you for the
purchase of a new home in the San Francisco area in an amount not to exceed the
unencumbered equity value in the purchase price (or construction and/or
renovation cost) of your new home, up to a maximum loan amount of $1 million. 
The loan shall have a term of ten (10) years, shall be secured by a mortgage on
the property and shall bear interest at the "applicable federal rate" as defined
in Section 1274(d) of the Internal Revenue Code, which interest shall be payable
quarterly in arrears.  This loan shall be forgiven over your period of service
hereunder at the rate of 0.83333% of the principal amount thereof per month;
provided that the aforementioned forgiveness shall cease upon the date of notice
of termination in accordance with Section 7 below, except as otherwise provided
in Section 8(d).
 
          (b)  REIMBURSEMENT OF RELOCATION EXPENSES.  All customary expenses
associated with the cost of your relocation to California shall be reimbursed by
the Company.  These expenses will include all moving and transportation costs of
the moving company of your choice, costs of trips to California for the purpose
of finding a new home, transportation costs for you and your family and for
household goods and automobiles, reimbursement of reasonable brokerage expenses
and other customary expenses associated with the selling of 
 
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<PAGE>
 
your present home, the reasonable transaction costs for the acquisition of 
your new home, the cost of appropriate temporary housing for you for up to 
six months.  In lieu of the reimbursement of relocation expenses provided by 
this Section 6(b), you may elect to receive an employment reimbursement bonus 
of One Hundred Thousand Pounds (L 100,000), payable five (5) business days 
after receipt by the Company of notice of your election.
 
     7.   TERM AND TERMINATION.
 
          (a)  TERM.  The period of your employment with the Company pursuant to
the provisions of this letter shall be for a continually renewing three (3) year
period commencing upon your start date with the Company, which we currently
expect to be not later than May 1, 1998 provided, however, that you agree to
become a service provider to the Company by providing reasonable consulting
services to the Company on and after the date you sign this Agreement.
 
          (b)  TERMINATION.  You may terminate your employment hereunder at any
time, with or without Good Reasons, as defined below, upon written notice to the
Company.  The Company may terminate your employment hereunder upon written
notice to you, with or without Cause as defined below.
 
          As used herein, "Good Reason" shall mean any of the following events
that are not consented to by you: (i) there has been a substantial diminution in
your duties and responsibilities hereunder, or the assignment to you of any
duties inconsistent in any respect with your position (including status,
offices, titles, and reporting requirements), authorities, duties, or other
responsibilities as in effect immediately prior to such assignment, or any other
action of the Company which results in a diminishment in such position,
authority, duties, or responsibilities  or any change in your direct reporting
relationship to the Board as a whole (a change in your title and duties to
Chairman of the Board will not be a change or diminution under this clause,
whether or not you retain the title of President); or (ii) the Company
headquarters shall move more than 30 miles from its present location in
Emeryville, California; or (iii) any of the following shall occur: (A) a
reduction by the Company in your base salary as the same shall be increased from
time to time; (B) the failure by the Company to provide you with compensation
and benefits at least equal (in terms of benefit levels and/or reward
opportunities) to those provided for under each compensation or benefit plan,
program, policy and practice as in effect (or as in effect hereafter, if
greater); (C) the failure of the Company to obtain a satisfactory agreement from
any other successor to the Company to assume and agree to perform this
Agreement; or (D) a material breach by the Company of its obligations under this
Agreement after notice in writing from you and a reasonable opportunity for the
Company to cure or substantially mitigate any material adverse effect of such
breach; provided, however, that no change in ownership or control of the
Company, including any transaction by which Novartis AG acquires additional
ownership of Company, including a Buy-Out Transaction as defined in the
Governance Agreement shall itself constitute Good Reason; and provided, further,
that an insubstantial and inadvertent action which is remedied by the Company
promptly after receipt of notice thereof given by you shall not constitute Good
Reason hereunder.  Your consent to any event which would otherwise constitute
"Good Reason" shall be conclusively presumed if you do not exercise your rights
under the first sentence of this Section 7(b) within 180 days of the event.
 
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<PAGE>
 
          As used herein, "Cause" shall mean any of the following events (i) any
willful misconduct on your part which has a material adverse effect upon the
business or affairs of the Company; or (ii) your conviction of a felony, or your
commission of any act of fraud against the Company or under federal or state
securities laws; or (iii) willful and continued failure by you to substantially
perform your duties as President and CEO (other than any failure resulting from
disability or from termination by you for Good Reason) as evaluated by a
majority of the Board and after written demand from the Board of Directors for
substantial performance is delivered to you, and you have failed to resume
substantial performance of your duties on a continuous basis within 30 days of
such notice.  Failure to perform your duties with the Company during any period
of disability shall not constitute Cause.
 
          (c)  DEATH OR DISABILITY.  The employment relationship created
hereunder shall immediately terminate upon your death or, at the election of you
or the Company,  upon disability which would preclude you from performing your
usual duties for the Company for a period in excess of ninety (90) consecutive
days or for a period in excess of ninety (90) days within any consecutive twelve
(12) month period.
 
          If your employment shall terminate all compensation and benefits other
than severance benefits described in Section 8 below, to the extent applicable,
shall immediately cease, except that you will be entitled to payment of your
salary through the date of termination and benefits under the Company benefit
programs and plans in accordance with their terms.  For the avoidance of doubt,
except as provided in Section 8, you will not receive bonus cash compensation or
other long-term compensation for the period in which termination occurs.
 
     8.   SEVERANCE BENEFITS IN CERTAIN EVENTS.
 
          If your employment shall be involuntarily terminated by the Company
other than for Cause, or if you terminate your employment for Good Reason, you
shall be entitled to receive as liquidated damages the following severance
benefits:
 
          (a)  CONTINUED COMPENSATION.  You shall continue to receive your base
salary at the rate in effect pursuant to Section 2(a) above at the time of your
termination of employment, for a period of three (3) years following your
termination date.  You may elect at any time following termination to require
that the Company prepay the unpaid balance of this amount, discounted to present
value at 7%, without thereby relieving you of your obligations under Section
1(d) above.
 
          (b)  BONUS.  You shall be entitled to receive in lieu of the bonus
provided in Section 2(b) an amount equal to three (3) times the greater of (x)
your targeted level bonus in the year of termination or (y) your highest bonus
under Section 2(b) in the preceding three years, in either case payable monthly
over the salary continuation period.  You may elect at any time following
termination to require that the Company prepay the unpaid balance of this
amount, discounted to present value at 7%, without thereby relieving you of your
obligations under Section 1(d) above.
 
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<PAGE>
 
          (c)  HEALTH CARE AND LIFE INSURANCE COVERAGE.  Continued health care
coverage under the Company's medical plan will be provided, without charge, to
you and your eligible dependents until the earlier of (i) three (3) years after
the effective date of your involuntary termination, or (ii) the first date that
you are covered under another employer's health benefit program providing
substantially the same or better benefit options to you without exclusion for
any pre-existing medical condition.  This coverage will be in lieu of any other
continued health care coverage to which you or your dependents would otherwise,
at your own expense, be entitled to in accordance with the requirements of Code
Section 4980B by reason of your termination of employment.  The Company will pay
the premium for continued life insurance coverage, if any, that you may have
elected under this Company's cafeteria benefit plan, subject to payment by you
of the portion of such premium not contributed by the Company under such plan,
until the earlier of three (3) years from the effective date of your separation
or your acceptance of employment with a successor employer.  If you make the
election set forth in the last sentence of either Section 8(a) or 8(b), the
benefits under this Section 8(c) shall terminate.
 
          (d)  TAX MATTERS.  All compensation described in this Section 8 will
be subject to the Company's collection of all applicable federal, state and
local income and employment withholding taxes.  The provisions of this Section 8
are in all events to be interpreted in such a manner as will avoid the
imposition of excise taxes under Section 4999 of the Internal Revenue Code, and
the disallowance of deductions under Section 280G(a) of the Internal Revenue
Code, with respect to any severance payment paid pursuant to this Section 8 and
you will cooperate reasonably with the Company in implementing this
interpretation.  Notwithstanding the foregoing, if any excise tax is imposed
under Section 4999 and/or deduction is lost under Section 280G(a), the Company
will bear the cost of the loss of deduction.
 
          (e)        REPATRIATION ASSISTANCE.  The Company recognizes that you
hold citizenship of South Africa and agrees to contribute reasonably to your
repatriation from the United States following a termination of your employment
under this Section 8.
 
 
          (f)    VESTING.  We will accelerate the immediate vesting of those 
stock options granted pursuant to Section 5(a) above that would otherwise 
have vested with continued employment during the next three (3) years 
following such termination.  You will have 90 days from the termination of 
your employment to exercise vested stock options in accordance with the terms 
of the Stock Option Plan.
 
     The foregoing provisions are intended to be liquidated damages, and
represent your and our sole and exclusive remedies for any damages caused by any
involuntary termination without Cause or voluntary termination for Good Reason,
the parties having agreed that it would be impractical or extremely difficult to
fix actual damages.
 
     9.   INDEMNIFY AND HOLD HARMLESS.
 
          Except to the extent inconsistent with the Company's Certificate of
Incorporation or bylaws, the Company will indemnify you and hold you harmless to
the fullest extent permitted by law with respect to your acts of service as an
officer and director of the 
 
                                       6
 
<PAGE>
 
Company.  The Company further agrees that you will be covered by directors' 
and officers' insurance policies with respect to your acts as an officer and 
director hereunder to the same extent as all other officers and directors 
under such policies, unless the Board, with your participation as a member of 
the majority thereof, determines not to maintain such policies.
 
     10.  MISCELLANEOUS.
 
          (a)  ARBITRATION.  Any dispute which may arise between you and the
Company with respect to the construction, interpretation or application of any
terms or provisions of this letter will be submitted to and resolved by
arbitration in San Francisco, California, in accordance with the rules of the
American Arbitration Association then in effect.  The arbitration shall be
before a single arbitrator agreed by the parties.  If we are unable to agree on
a single arbitrator within 60 days of the date either of us demands arbitration,
then a single arbitrator shall be designated by the San Francisco office of the
American Arbitration Association, which arbitrator shall be experienced in
executive employment and compensation matters.  The arbitration decision shall
be final and binding to the fullest extent permitted by law and enforceable by
any court of competent jurisdiction.  The prevailing party shall be entitled to
recover its costs and expenses as determined by the arbitration.
 
          (b)  TAXES.  You agree that you will work reasonably with the Company
to structure your salary and bonus compensation and your restricted stock grant
in such a fashion as to qualify for tax deductibility under Section 162(m) of
the Internal Revenue Code while preserving the economic benefit intended to be
received by you.  Additionally, all compensation paid to you under this letter
shall be subject to the Company's collection of all applicable federal, state
and local income and employment withholding taxes.
 
          (c)  GOVERNING LAW.  This Agreement shall be construed and enforced in
accordance with and be governed by the laws of the State of California.
 
          (d)  ENTIRE AGREEMENT.  This Agreement sets forth the entire Agreement
and understanding between you and the Company, and supersedes any other
negotiations, agreements, understandings, oral agreements, representations and
past or future practices whether written or oral.
 
          (e)  NOTICES.  All notices required by this Agreement shall be given
in writing either by personal delivery or by first class mail, return receipt
requested, to the then most current address of the parties notified to the
other.  Notice given by mail shall be deemed given five (5) days following the
date of mailing.
 
          (f)  SUCCESSORS.  This Agreement shall be binding and inure to the
benefit of the Company and its successors.
 
          (g)  MODIFICATION OR WAIVER.  This Agreement may not be amended,
modified, changed or discharged in any respect, except as agreed in writing.  No
term or condition of this Agreement will be deemed to have been waived, nor will
there be any estoppel to enforce any provisions of this Agreement, except by a
statement in writing signed by the party against whom enforcement of the waiver
or estoppel is sought.  Any written waiver will not be deemed a continuing
waiver unless specifically stated, will operate only as to 
 
                                       7
 
<PAGE>
 
the specific term or condition waived and will not constitute a waiver of 
such term or condition for the future or as to any act other than that 
specifically waived.
 
          (h)  ASSIGNMENT.  This Agreement is not assignable, in whole or in
part, by any party without the written consent of the other party.
 
          (i)  SEVERABILITY.  To the extent that any provision of this Agreement
shall be determined to be invalid or unenforceable, the invalid or unenforceable
portion of such provision will be deleted from this Agreement, and the validity
and enforceability of the remainder of such provision and of this Agreement will
be unaffected.  In furtherance of and not in limitation of the foregoing, it is
expressly agreed that should the duration of or geographical extent of, or
business activities covered by, the noncompetition covenant contained in Section
1(d) be determined to be in excess of that which is valid or enforceable under
applicable law, then such provision will be construed to cover only that
duration or extent, or those activities which may validly or enforceably be
covered.  You acknowledge the uncertainty of the law in this respect and
expressly stipulate that this Agreement will be construed in a manner which
renders its provisions valid and enforceable to the maximum extent (not
exceeding its express terms) possible under applicable law.
 
          (j)  Your employment by the Company is subject to all applicable law,
including your ability to be employed in the United States under the U.S.
Immigration and Naturalization Act.  The Company will provide assistance in
processing applications for appropriate immigration status.
 
          We are all very pleased at the prospect that you will be joining the
Company.  If the foregoing is acceptable to you, please sign the enclosed copy
of this letter and return it to me.
 
                                   Very truly yours,
                              
                                   CHIRON CORPORATION
                                   
                                   
                                   By:  /s/ Edward E. Penhoet
                                      -------------------------------
                                       Edward E. Penhoet, Ph.D.
                                       President and Chief Executive Officer
 
Agreed:  /s/ Sean P. Lance
         --------------------
             Sean P. Lance
 
Dated:   March 18, 1998