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Exhibit 10.626

[CHIRON LETTERHEAD]

July 5, 2004

CONFIDENTIAL

Ursula B. Bartels
(Address)

Dear Ursula:

        We are pleased to confirm our offer of employment for you to join Chiron
Corporation.

        Your employment offer with Chiron is as Vice President and General
Counsel, reporting to me. Your starting salary will be $400,000 per annum, paid
bi-weekly.

        You will be eligible to participate in our stock option program. Subject
to the approval of the Board of Directors, you will be awarded a stock option
grant to purchase 200,000 shares of Chiron common stock. The options vest fully
over a four-year period, with the first 25% of the shares vesting at the
one-year anniversary. Shares vest on a pro-rated monthly basis over the
remaining three years of the vesting period. The exercise price of the option
will be set at the fair market value (as defined in the Option Plan) of a share
of Chiron stock on the date the option is approved. We expect the options will
be approved and effective within 90 days of your employment date. You are also
eligible to participate in Chiron's Long Term Incentive Program for the 3 year
performance cycle beginning 2004. Your target award is 7,500 restricted share
rights. Depending upon the performance of the company against predetermined
goals, you may receive up to 150% of your target shares. The performance period
ends December 31, 2006 and the Compensation Committee of the Board will
determine the level of performance goal achievement and the number of shares, if
any, you will be entitled to receive, assuming you are continuously employed for
the period. Details of the plan will be furnished to you when you start.

        Also, on an annual basis you may be eligible for future stock option
grants based on your performance. These annual grants are typically granted in
February and are pro-rated based on your hire date.

        You will also be eligible to participate in Chiron's Annual Incentive
Plan (AIP). The Plan links corporate, business/functional unit and individual
performance goals to support the accomplishment of Chiron's business goals. The
Plan, as structured, has an incentive range for your position from 0% to 150%
with a target of 75% of base pay. Ordinarily, plan participants hired between
January 1 and September 30, 2004 are eligible to receive a pro-rated bonus, paid
in the first quarter of 2005.

        You are eligible for Chiron's relocation assistance program. Prudential
Relocation administers the program for Chiron and Joel Spokas is Chiron's
account representative with Prudential. He can provide you with details of the
program, but I will send you a summary by separate mailing. His telephone number
is 800-210-0299 and his e-mail address is joel.spokas@prudential.com.

        As an inducement for you to join Chiron, we will pay you a bonus of
$250,000, payable within your first 2 weeks of employment. This is also
intended, in part, to assist in your relocation.

        The information sheets following your offer letter contain some of the
highlights of Chiron's benefits programs. You should note that, while this offer
is being made under the terms of our current benefits and compensation programs,
changes do occur from time to time and any system-wide changes that occur will
apply to you as well.

        Some of these programs are effective immediately upon your employment,
while others are dependent upon established enrollment periods. Your group
medical insurance benefits generally start on the first day of the month
following your date of hire. If, however, your date of hire is the first
business day of the month, your coverage becomes effective on that date. Note
also that, with few

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exceptions, Chiron extends benefits coverage to qualified family members,
including opposite- and same-sex domestic partners. You will have an opportunity
to learn more about your benefits and complete the enrollment paperwork at a
benefits orientation. The schedule will be provided for you at new hire
orientation.

        If you have an employment authorization document (EAD) issued by the
Immigration and Naturalization Service, please contact Inverleith Fruci in
Emeryville, California at 510-923-3733.

        Your employment with Chiron is not for any specified term and can be
terminated at any time for any reason by you or Chiron.

        Your employment pursuant to this offer is contingent on the following:

•Your signing the Chiron Employee Invention and Confidential Information
Agreement.

•Your ability, as required by federal law, to establish your employment
eligibility as a U.S. citizen, a U.S. lawful permanent resident, or an
individual specifically authorized for employment in the U.S. by the Immigration
and Naturalization Service. Please read the enclosed notice specifying the
documentation required in this regard.

•Satisfactory completion of a reference check.

•A baseline blood sample for archival storage may be required for some lab
positions.

        This letter constitutes the terms of your employment with us and
supersedes any prior representations or agreements, whether written or oral. A
duplicate original of this offer letter is enclosed for your records. To accept
employment at Chiron under the terms listed above, please sign and date this
letter and return to Chiron.

        The Executive Committee and I believe you are a great fit for our
requirements at Chiron. Your experience will allow you to make excellent
contributions to the success of Chiron as General Counsel and in your capacity
as a member of the Executive Committee of the company. We look forward to your
acceptance of our offer and your taking up employment mid-August.

        If you have any questions in the meantime, please feel free to contact
me or Linda Short.

Sincerely,

CHIRON CORPORATION

/s/ Howard H. Pien
Howard H. Pien
Chairman and Chief Executive Officer

Enclosures

        Please indicate your understanding of the terms of this offer and your
acceptance of this offer, along with your confirmed start date, by signing this
letter and filling out the attached packet of forms. In order to expedite the
processing of your employment records, please return the letter and completed
forms to the Chiron Human Resources Department in the enclosed postage paid
envelope as soon as possible. Please keep the copy of your offer letter.

/s/ Ursula B. Bartels

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Ursula B. Bartels   7/7/04

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Date

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CHIRON CORPORATION
EMPLOYEE BENEFITS FOR EXECUTIVES AS OF 1/1/2004

INSURANCE BENEFITS    
MEDICAL INSURANCE
 
Chiron offers several medical plans to eligible employees and their eligible
dependents (spouse, domestic partner and/or children). All medical plans include
prescription and vision benefits. Employee contributions are pretax.
DENTAL INSURANCE
 
Chiron offers 2 dental plans to eligible employees and their eligible dependents
(spouse, domestic partner and/or children). Employee contributions are pretax.
LIFE INSURANCE
 
Eligible employees can purchase coverage for themselves of $10,000, or 1, 2, 3,
4 or 5 times annual salary. The maximum coverage is $1.6 million. Employee
contributions are pretax.
DEPENDENT LIFE INSURANCE
 
Eligible employees can purchase life insurance coverage for eligible dependents
on an after-tax basis. Options include $10,000, $20,000 or $50,000 for
spousal/domestic partner coverage and $5,000 or $10,000 for children.
ACCIDENTAL DEATH & DISMEMBERMENT
 
Eligible employees can purchase coverage for themselves of 1, 2, 3, 4 or 5 times
annual salary. The maximum coverage is The maximum coverage is $1.6 million.
Employee contributions are pretax.
TRAVEL ACCIDENT
 
Chiron provides Travel Accident Insurance coverage of 3 times annual salary at
no cost to employees. This provides 24-hour coverage for travel to and from work
or on company business.
EXECUTIVE SHORT-TERM DISABILITY
 
Through the short-term disability program, Chiron provides eligible employees
with 100% of salary for the first 30 calendar days less state disability
payments, and 80% of salary for the next 150 days of disability less state
disability payments. Chiron provides this coverage at no cost to the employee.
LONG-TERM DISABILITY
 
For eligible employees disabled for more than 180 days, Chiron provides 60% of
monthly salary up to $18,000. Chiron provides this coverage at no cost to the
employee. Eligible employees also have the option to purchase an additional
62/3% of monthly salary up to $20,000 on a pretax basis.
WORKERS' COMPENSATION
 
All employees are covered by Chiron's Workers' Compensation insurance.
INVESTMENT PROGRAMS
 
 
401(k) PLAN
 
Participation in the Chiron Corporation 401(k) plan is available to eligible
employees on the date of hire. Upon hire, an employee can enroll immediately at
5% pre-tax deferral into a company selected investment fund. Upon receiving a
personal identification number (PIN), an employee may designate pre-tax
contributions of 1% to 25% of salary and direct contributions to any of nine
investment funds. The Company provides a maximum 4% match on employee
contributions and the match is vested immediately.      

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SUPPLEMENTAL EXECUTIVE RETIREMENT PLAN
 
All officers of Chiron at the level of Executive or above are eligible to
participate in the Supplemental Executive Retirement Plan (SERP). SERP is
designed to allow participants to make before-tax contributions for retirement
that would otherwise exceed the Internal Revenue Service's maximum contribution
limits allowed in Chiron's 401(k) plan. Similarly, SERP allows the company to
match contributions toward retirement in excess of the 401(k) plan's limits.
EMPLOYEE STOCK PURCHASE PLAN
 
Eligible employees may purchase Chiron stock at a discount below market price.
Participation in the plan depends upon the employee's date of hire. Employees
can contribute to this program from 3% to 15% of their base salary on an
after-tax basis.
TIME-OFF BENEFITS
 
 
EXECUTIVE TIME OFF
 
Each January 1, executive employees receive an allocation of 4 weeks of time
off. The allocation may be taken in whole or in part anytime throughout the
year. Any unused time at the end of the year will not accrue from year to year.
No reporting of time off is needed. There will be no pay out for unused time off
either at the end of the year nor at termination.
FIXED HOLIDAYS
 
Eligible employees receive up to 8 paid holidays per year.
LEAVE OF ABSENCE
 
Chiron grants leaves of absence to accommodate employees' medical/maternity
disability and family or personal needs.
OTHER BENEFITS
 
 
FLEXIBLE SPENDING
 
Chiron offers Health Care and Dependent Care reimbursement accounts to eligible
employees. These accounts give employees the advantage of paying for eligible
health and dependent care expenses with pretax dollars.
EMPLOYEE ASSISTANCE PROGRAM
 
Chiron provides a confidential counseling and referral service to all employees
and their families.
EDUCATIONAL ASSISTANCE
 
Regular full-time employees are eligible, after 6 months of service, for up to
$5,250 per year in reimbursement of tuition, textbooks, and lab expenses for
classes directly related to their jobs.
CREDIT UNION
 
Regular full-time and part-time employees and members of their families or
household are eligible to join.
PREPAID LEGAL
 
A Prepaid Legal Plan is available to eligible employees to provide affordable,
basic legal services. Employees pay for this benefit on an after-tax basis.
DEPENDENT CARE CONNECTION
 
Eligible employees are provided with this counseling, education and referral
service which provides information regarding child care, elder care, adoption
and prenatal planning.

NOTE: Eligible employees are defined as regularly scheduled to work 20 hours or
more per week. Coverage for most benefits is effective the first of the month
following date of hire, or immediately if the date of hire is the first business
day of the month, unless otherwise noted.

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Exhibit 10.626

CHIRON CORPORATION EMPLOYEE BENEFITS FOR EXECUTIVES AS OF 1/1/2004