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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:

1.You voluntarily terminate employment, unless you already received notice of a
qualifying involuntary termination and Chiron determines in its sole discretion
that earlier voluntary termination is in the best interests of Chiron;

2.You are discharged for reason other than workforce reduction or job
elimination (including, but not limited to, poor performance, gross negligence
or cause (including, but not limited to, any act of dishonesty, willful
misconduct, fraud or embezzlement or any unauthorized disclosure of confidential
information or trade secrets or violation of company policy or rules), whether
or not you had already received notice of a qualifying termination;

3.You either continue employment or are offered and decline employment with a
Chiron Company or any employer that acquires all or any portion of the assets or
operations of a Chiron Company or ceases to be a Chiron Company;

4.You are covered by any other severance pay plan or arrangement or by an
employment agreement, unless and to the extent that Chiron agrees in writing;
provided that in no event will benefits be payable under this Plan to the extent
that they would duplicate benefits payable under such other plan, arrangement or
agreement.

5.You are on a leave of absence (including disability leave), except to the
extent permitted by Chiron in its sole discretion, or you die before you receive
a severance payment under the plan;

6.You are not classified by a Participating Company as an Executive Employee
(whether or not such classification is subsequently deemed proper by a
government agency or court); or

7.You are employed by a Chiron Company or a work unit that the Plan
Administrator determines is not covered by the Plan.

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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.

Base salary is calculated at the annualized rate paid for the payroll period
immediately before the Executive Employee's notice of termination, including any
pre-tax amount the Executive Employee elected to have contributed to a 401(k)
plan or to a Section 125 plan.

Bonus is the Executive Employee's target Annual Incentive Plan ("AIP") bonus
applicable for the period immediately before the Executive Employee's notice of
termination.

A year of service means each 12 month period of employment with a Participating
Company, beginning with the Executive Employee's most recent hire date. Years of
service will not include service with a company before it becomes a
Participating Company unless otherwise specified by Chiron.

        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 in Exhibit B will
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 A

PARTICIPATING 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:

PowderJect Vaccines, Inc.

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EXHIBIT B

Executive Employee Benefits Which Continue During Salary Continuation

•Medical/Dental/Vision Coverage

•Reimbursement accounts (i.e., medical care account, dependent care account)

•Group Life Insurance Plans (i.e., life insurance, AD&D, dependent life
insurance)

•Prepaid Legal

Executive Employee Benefits Which Do Not Continue During Salary Continuation

•Business Travel Accident

•Workers Compensation

•Vacation and Sick Leave Accruals

•401(k) Plan

•Employee Stock Purchase Plan

•Stock Options

•Short and Long Term Disability (including Supplemental Long Term Disability
Insurance)

•Other Benefits Not Listed Above

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QuickLinks

Exhibit 10.507

CHIRON CORPORATION EXECUTIVE OFFICER SEVERANCE PLAN ( Effective December 1, 2003
)
ELIGIBILITY
HOW THE PLAN WORKS
FAILURE TO EXECUTE A RELEASE
OTHER IMPORTANT INFORMATION
STATEMENT OF ERISA RIGHTS
EXHIBIT A PARTICIPATING COMPANIES
EXHIBIT B