Exhibit 10.507

CHIRON CORPORATION    EXECUTIVE OFFICER SEVERANCE PLAN    (Effective December 1, 2003)

The
purpose of this Chiron Corporation Executive Officer Severance Plan is to assist full-time executive employees who are in Salary Grades E3 and above, as may be adjusted
from time to time (excluding the President and Chief Executive Officer) (the "Executive Employees"), who as a result of workforce reduction or job elimination, lose their positions with the
Corporation or any of its subsidiaries (each referred to as a "Chiron Company") which is at the time of termination designated as a Participating Company in Exhibit A, as attached hereto from time
to time (each referred to as a "Participating Company"). This plan applies only to U.S.-based Executive
Employees.

The
plan is effective for terminations of employment that occur on or after December 1, 2003.

ELIGIBILITY

If
you are an Executive Employee of a Participating Company, you will be eligible for executive severance pay if your active employment is involuntarily terminated by your employer
because of a workforce reduction or a job elimination on or after December 1, 2003 and before this plan is terminated. Except to the extent that severance is required by statute, this voluntary
executive severance pay plan is in consideration of your signing a separation agreement and release which releases Chiron from any legal claims you may have against Chiron Companies, and their
employees and agents.

You
are not eligible for Executive Employee severance pay under the plan if:

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HOW THE PLAN WORKS

Executive Employees covered under the plan will receive eight (8) weeks of paid advance notice of a qualifying termination. The notice may be working or
non-working, at the discretion of the supervisor; however, at least 6 weeks must be non-working. Termination of employment occurs at the end of this advance notice period.
Following termination, a qualifying participant who properly executes a release of claims against Chiron Companies and their employees and agents in the form provided by Chiron will receive the
following benefits:

1.Cash Severance Amount.Cash severance will be paid equal to 6 weeks base salary and bonus per year of
service. However, the total period of advance notice and severance will not be less than 26 weeks and cannot be longer than 2 years. The rate of severance pay will be determined by dividing
annual base salary by 52 to determine the amount of weekly salary and adding target bonus divided by 52.



2.Election Between Lump Sum and Continuation of Salary and Benefits.An Executive Employee who qualifies for
severance benefits may elect to receive those benefits either in a lump sum following termination of employment or in the form of salary continuation pursuant to the Participating Company's normal
payroll practice, beginning after termination of employment. If an Executive
Employee elects to receive severance in the form of salary continuation then those Executive Employee benefits listed inExhibit Bwill continue
during the period of salary continuation, provided that the Executive Employee maintains his or her Executive Employee contributions and the Participating Company from which the Executive Employee
terminates offers such benefits to its active Executive Employees. If an Executive Employee elects a lump sum severance payment, no benefits continue after termination of employment, except as may
otherwise be provided under the relevant benefit plans for terminations of employment generally.

3.Reemployment Outside of Chiron.Should an Executive Employee find other work outside of the Chiron Companies
before any severance benefit has been paid under the plan then, in lieu of any other benefits under the plan, the Executive Employee will be paid a lump sum cash payment equal to one-half
of the cash severance pay that would have been paid had the Executive Employee not begun such work. If the Executive Employee had elected and already started to receive salary continuation payments,
then the Executive Employee's salary continuation and benefits will stop at the point this new work begins and the lump sum cash payment will equal one-half of the remaining salary
continuation that would have been paid thereafter had the Executive Employee not begun such work. An Executive Employee who is re-employed with a non-Chiron company will not be
required to return any amount already received under the plan.

4.Reemployment With a Chiron Company.Should an Executive Employee find work with a Chiron Company before any
severance benefit has been paid under the plan, then no benefit will be payable under the plan. If salary continuation had already begun under the plan, then salary

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continuation
and benefits will stop and no lump sum severance will be paid, but the Executive Employee will again be eligible for benefits under this plan in the event of any subsequent qualifying
termination of employment while the plan is in effect. If an Executive Employee is reemployed by a Chiron Company after receiving a lump sum severance payment but before the end of the period for
which the Executive Employee would have received salary continuation had he or she not elected a lump sum, Chiron will require repayment of that portion of the lump sum attributable to the remainder
of that period as a condition of reemployment.

5.Tuition Reimbursement.All Executive Employees who have classes in progress for which Chiron tuition
reimbursement would normally apply will be provided the benefit in accordance with Chiron policy.

6.Stock Options.The effect of termination of employment on an Executive Employee's stock option will be
determined by the terms of the option agreement relating to that option. The following describes the effect of termination of employment under most Chiron stock options for most types of qualifying
terminations: An Executive Employee who has unvested stock options when notice of termination is given will continue to vest in those options during the period of advance notice, but not during any
subsequent salary continuation period. A stock option that is unvested at the end of the advance notice period will be cancelled. Executive Employees with vested stock options at the end of the
advance
notice period must exercise those options within three months after the end of the notice period or the options will be cancelled. Executive Employees with option grants that contain the retirement
feature and who qualify for the retirement feature under the terms of those grants will have extended vesting and exerciseability according to the terms of each grant. Continued employment with a
company that ceases to be a Chiron Company is not considered continued employment for purposes of stock option vesting or exercisability.

7.Bonuses and Commissions.This plan will not affect any Executive Employee's right to bonuses or commissions
fully earned before the end of the notice period in accordance with the terms of the applicable bonus or commission plan.

8.Job Search Assistance.Executive Employees will be provided with the additional benefit of job search
assistance. The job search assistance will be provided as follows:

Full
outplacement service including office space and clerical support for a period of six months.

FAILURE TO EXECUTE A RELEASE

All benefits provided under the plan after the end of the notice period (including any lump sum severance payment, continuation of salary and\or benefits) are in
consideration of your execution of a release of claims against all Chiron companies and their employees and agents in the form provided by Chiron. If you do not properly execute such a release, you
will not be entitled to any of the benefits of this plan, other than the period of advance notice.

OTHER IMPORTANT INFORMATION

1.Plan Administrator's Discretion.As the Plan Administrator, Chiron will have full discretionary authority to
administer and interpret this plan, including, but not limited to discretionary authority to determine eligibility for benefits under the plan and the amount of benefit under the plan. Chiron may
delegate administrative duties to Chiron personnel. Any such delegation will carry with it the full discretionary authority of the Administrator to carry out these duties. Any determination by the
Administrator or its delegate will be final and conclusive upon all persons.

2.Non-Assignability.Benefits under the plan are not assignable and will be paid when due from the
general assets of Chiron or the Participating Company that employed the eligible Executive Employee.

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3.Confidentiality Agreements.Benefits under the plan are conditioned upon your compliance with any
confidentiality agreement you have entered into with a Chiron Company.

4.Coordination With Mandated Benefits.Any advance notice or benefits provided under this plan shall, to the
fullest extent permitted by law, be considered to be in satisfaction of, rather than in addition to, any federal, state or local requirement to provide advance notice or severance-type
benefits. To the extent that notice or benefits provided under this plan cannot be considered in satisfaction of any such requirement, the amount of notice and benefits otherwise payable under this
plan shall be reduced by the amount of notice and benefits that are required to be given by federal, state or local law.

5.Claims Disputes.If you believe you are entitled to a greater benefit under the plan, you may submit a
signed, written application to the Plan Administrator within 90 days of your termination. You will generally be notified of the approval or denial of this application within 90 days of
the date that the Plan Administrator receives the application. If your claim is denied, the notification will state specific reasons for the denial and you will have 60 days to file a
signed, written request for a review of the denial with the Plan Administrator. This request should include the reasons you are requesting a review, facts supporting your request and any other
relevant comments. The Plan Administrator will generally make a final, written determination of your eligibility for benefits within 60 days of receipt of your request for review.

6.Amendment and Termination.Chiron reserves the right, in its sole and unlimited discretion, to amend or
terminate the plan at any time and in any manner.

7.Withholding Taxes.Chiron will withhold taxes and other payroll deductions from any severance payment.

8.No Right to Employment.This plan does not provide you with any right to continue employment with any Chiron
Company or affect the right of you or your Chiron Company employer to terminate your employment at any time, with or without cause; provided that nothing herein shall adversely affect any rights under
a written employment agreement executed by the parties thereto.

9.ERISA Governs.This plan is governed by the Employee Retirement Income Security Act of 1974, as amended
("ERISA") and, to the extent applicable, the laws of the State of California. This Plan is not intended to be funded for purposes of ERISA.

10.Plan Year.The Plan Year is the calendar year.

11.Agent for Service of Process.The Agent for service of legal process for this plan is the General Counsel of
Chiron Corporation. Process may also be served on Chiron. Chiron's employer identification number is 94-2754624. Chiron Corporation's address and telephone number are 4560 Horton Street,
Emeryville, CA 94608, (510) 655-8729.

STATEMENT OF ERISA RIGHTS

If you are eligible for benefits under the plan, you are entitled to certain rights and protections under ERISA. You may examine (without charge) all plan
documents, including documents filed with the U.S. Department of Labor, at the Human Resources Department, Chiron Corporation, 4560 Horton Street, Emeryville, CA. You may obtain copies of all plan
documents and other plan information upon written request to the Plan Administrator. (The document containing this statement constitutes both the plan document and the summary plan description.) A
reasonable charge may be made for such copies.

In
addition to creating rights for certain employees under the plan, ERISA imposes duties upon the people who are responsible for the operation of the plan. The people who operate the
plan (called "fiduciaries") have a duty to do so prudently and in the interest of Chiron Executive Employees who

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are
covered by the plan. No one, including Chiron or any other person, may fire you or otherwise discriminate against you in any way to prevent you from obtaining a benefit to which you are entitled
under the plan or from exercising your rights under ERISA. If your claim for a severance benefit is denied, in whole or in part, you must receive a written explanation of the reason for the denial and
you have the right to a review of the denial.

Under
ERISA, there are steps you can take to enforce the above rights. For instance, if you request materials and do not receive them within 30 days, you may file suit in a
federal court. In such a case, the court may require the Administrator to provide the materials and to pay you up to $100 a day until you receive the materials, unless the materials were not sent
because of reasons beyond the control of the Administrator. If you have a claim that is denied or ignored, in whole or in part, you may file suit
in a state or federal court. If it should happen that plan fiduciaries misuse the plan's assets (if any), or if you are discriminated against for asserting your rights, you may seek assistance from
the U.S. Department of Labor, or you may file suit in a federal court.

The
court will decide who will pay court costs and legal fees. If you are successful, the court may order the person you have sued to pay these costs and fees. If you lose, the court may
order you to pay these costs and fees, for example, if it finds that your claim is frivolous.

If
you have any questions about the plan you may contact the Plan Administrator. If you have any questions about this statement or about your rights under ERISA, you may contact the
nearest area office of the U.S. Labor-Management Services Administration, Department of Labor.

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EXHIBIT APARTICIPATING COMPANIES

Commencing December 1, 2003 (or as otherwise noted) and continuing thereafter until the Plan Administrator determines otherwise or the plan terminates, the
following Chiron Companies shall be Participating Companies in the plan:

All
Chiron Companies with U.S.-based Executive Employees, except:



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EXHIBIT B

Executive Employee Benefits Which Continue During Salary Continuation



Executive Employee Benefits Which Do Not Continue During Salary Continuation



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