Document:

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                                                                   Exhibit 10.21

                          BioSource International, Inc.
                                 540 Flynn Road
                           Camarillo, California 93012

September 18, 2000

PRIVATE AND CONFIDENTIAL

Mr. Russell D. Hays
9 Stratford Way
Lincoln, MA 01773

Dear Russ:

Reference is made to that certain letter agreement dated August 2, 2000 (signed
by you on August 9, 2000) between and among BioSource International, Inc.
("BioSource"), you and Genstar Capital, LLC. For good and valuable
consideration, the receipt and sufficiency of which is hereby acknowledged, the
parties agree that the first full paragraph appearing on the top of page 3 of
the letter agreement is hereby amended to read as follows:

      Should there be a "change of control," all stock options that have been
      granted to you will vest immediately. If you are terminated for any reason
      other than for cause you will receive one year's salary and health care
      benefits over the regular pay period.

      A "change of control" means (i) any person or entity (or group of related
      persons or entities acting in concert) unaffiliated with Genstar Capital
      LLC ("Genstar") shall acquire shares of capital stock of the Company
      entitled to exercise 40% or more of the total voting power represented by
      all shares of capital stock of the Company then outstanding; or (ii) the
      Company shall enter into an agreement to sell or otherwise transfer all or
      substantially all of its assets or enter into an agreement to merge,
      consolidate or reorganize with any other corporation or entity, as the
      result of which less than 60% of the total voting power represented by the
      capital stock or other equity interests of the corporation or entity to
      which the Company's assets are sold or transferred or surviving such
      merger, consolidation or reorganization shall be held by the persons and
      entities who were holders of common stock of the Company on the date
      immediately prior to such transaction; or (iii) the Company shall issue to
      a party or parties unaffiliated with Genstar Capital LLC ("Genstar")
      otherwise than on a pro rata basis additional shares of capital stock
      representing (after giving effect to such issuance) more than 40% of the
      total voting power of the Company; or (iv) the persons who were the
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      directors of the Company as of September 18, 2000 shall cease to comprise
      a majority of the Board of Directors of the Company.

      "For cause" means delivery of written notice of termination by the Company
      "for cause," by reason of: (i) any act or omission knowingly undertaken or
      omitted by you with the intent of causing damage to the Company, its
      properties, assets or business or its stockholders, officers, directors or
      employees; (ii) any act by you involving a material personal profit to
      you, including, without limitation, any fraud, misappropriation or
      embezzlement, involving properties, assets or funds of the Company or any
      of its subsidiaries; (iii) your consistent failure to perform your normal
      duties or any obligation, as directed by the Board; (iv) your conviction
      of, or pleading nolo contendere to, (A) any crime or offense involving
      monies or other property of the Company; (B) any felony offense; or (C)
      any crime of moral turpitude; or (v) your chronic or habitual use of drugs
      or consumption of alcoholic beverages.

In all other respects, the letter agreement shall remain unchanged.

Sincerely,

BIOSOURCE INTERNATIONAL, INC.

/s/ Jean-Pierre L. Conte
-----------------------------
Jean-Pierre L. Conte
Director

Accepted this 18th day of September, 2000 Accepted this 18th day of September,
2000

/s/ Jean-Pierre L. Conte                /s/ Russell D. Hays
-----------------------------           ----------------------------
Jean-Pierre L. Conte                    Russell D. Hays
Managing Director
Genstar Capital, LLC<PAGE>

                                                                   EXHIBIT 10.22

                            BIOSOURCE INTERNATIONAL
                                 820 Flynn Road
                              Camarillo, CA 93012

August 18, 2000

PRIVATE AND CONFIDENTIAL

Mr. George Uveges
207 Willowbrook Drive
Wayland, MA 01776

Dear George:

This letter extends an offer to you to become the Chief Operating Officer of
BioSource International, Inc. ("BioSource" or the "Company"). After several
meetings and discussions, as well as your exemplary work at NEN Life Sciences,
Inc., Russ Hays, Jim Chamberlain, Robert Weltman and I believe you bring exactly
the drive and experience required to lead BioSource. Please consider this
document a conveyance of the basic terms and conditions of our offer. If,
however, you feel that a more explicit document is required in order to capture
some of the specifics of our relationship, we can work together in crafting that
instrument to our mutual satisfaction.

Position Responsibility
-----------------------

You shall serve as Chief Operating Officer of the Company. As the Chief
Operating Officer, you will report to the Chief Executive Officer on executive
matters and will have a significant role in developing the strategic direction
and vision of BioSource, and carrying out the day-to-day activities of the
Company consistent with Board approved strategies and budgets.

Your official start date of these duties will be September 18, 2000.

Salary
------

The base salary of your position will be two hundred and twenty thousand dollars
($220,000) per annum, which shall be paid in installments on a bi-weekly basis.
The Company shall review your performance on each anniversary of your date of
hire and at its sole discretion may increase your base salary for the position
based on the Board's evaluation of your performance and the financial condition
of the Company at that time.
<PAGE>

Mr. George Uveges
August 18, 2000
Page 2

Bonus and Incentive Compensation
--------------------------------

A copy of the draft executive bonus plan entitled "Annual Incentive Plan" will
be faxed to you in the next few days and we will be pleased to review it with
you. Central to that discussion will be the Company's budgeted sales and EBITDA,
performance measurements on which the incentive plan is based. We have given to
you management's 2000 budget and incentive plan. We must agree on the revised
performance targets for the 2001 fiscal year by December 15, 2000. As you know,
the design of our plans contain the following key elements:

      1)    Threshold for payment of any bonuses is corporate performance of
            slightly below the sales and EBITDA budget.

      2)    Target bonuses are paid upon achieving 100% of the sales and EBITDA
            budget.

      3)    Bonuses are not capped.

      4)    The plan incorporates an increasing percentage of payout as Company
            performance progresses from threshold to budget and beyond.

      5)    At 100% of budget, your bonus would be 40% of salary.

Equity Participation
--------------------

You have been invited to invest in BioSource. We have offered you the
opportunity to invest up to one million dollars ($1,000,000) at the Market
Price. The closing price for BioSource stock on August 18, 2000 was $16.75 per
share. Market Price for purposes of this letter shall mean the lower of the
Closing Price for BioSource common stock as reported on NASDAQ (i) on the date
you sign this letter and accept this offer or (ii) on the date you officially
start. You will be granted registration rights in connection with this
investment pari passu with the Series A Preferred stock of the Company. All
shareholders will be subject to future dilution for such things as additional
grants under the stock option plan. Your investment is subject to the
availability of applicable exemptions for such investment under the Securities
laws and all NASDAQ rules.

Stock Options
-------------

The Board of Directors of BioSource has established a Stock Option Plan for the
management and employees of the Company. Coincident with your employment, the
Company will grant you 200,000 options with an exercise price at the Market
Price. The options will be exercisable for ten (10) years and will have a four
(4) year vesting schedule in equal twenty-five percent (25%) increments on each
anniversary date of the grant.
<PAGE>

Mr. George Uveges
August 18, 2000
Page 3

Should there be a significant change whereby a Company or investor unaffiliated
with Genstar Capital LLC ("Genstar") acquires directly or indirectly 40% or more
of the fully diluted shares of the Company ("Change of Control"), all stock
options that have been granted to you will vest immediately. If you are
terminated for any reason other than cause you will receive one year's salary
and health care benefits over the regular pay period.

Benefits
--------

You shall be entitled to participate in the Company's benefit plans, including
group health and medical insurance plans, as they are adopted from time to time
beginning on the first day of the month following your employment. A description
of these plans will be sent under separate cover and someone will be available
to provide further explanation. You shall be entitled to four (4) weeks paid
vacation per year.

401K Savings Plan
-----------------

You will be eligible to join the 401K savings plan of BioSource.

Expense Reimbursement
---------------------

The Company will reimburse you for reasonable and necessary business expenses
incurred while performing your duties as described in this letter, subject to
the policies, procedures and controls that the Company may from time to time
adopt. Expense reimbursement will be subject to the approval of one of the
directors of the Company.

Relocation Cost Reimbursement
-----------------------------

It is understood and agreed that you will commute from your home for the first
six months of your employment after which, the Company will reimburse you for
reasonable expenses related to your move to whatever site is chosen to be the
Company's headquarters, including trips by your wife to research housing and
interim housing while you are in Camarillo, CA, until you find permanent
housing. Reasonable expenses will include but not be limited to physical moving
costs, loan points, real estate fees, and closing costs all grossed up to the
maximum rate. At the appropriate time please send us a proposed budget for your
relocation costs.

Indemnification
---------------

The Company will indemnify you to the fullest extent possibly by law for any
liability incurred as a result of the conduct of your duties as an employee of
BioSource.
<PAGE>

Mr. George Uveges
August 18, 2000
Page 4

George, we believe that BioSource will provide an excellent vehicle for your
next challenge. You have pointed out several growth and profitability
improvement opportunities for us and we believe that you are the right person to
lead and grow the business of BioSource. Needless to say, we are excited that
you will be joining BioSource as a partner.

We have extended this offer to you to become the Executive Vice President,
Administration of the Company in good faith, and we are looking forward to a
long and close working relationship with you.

Sincerely,

BIOSOURCE INTERNATIONAL

/s/ Jean-Pierre L. Conte

Jean-Pierre L. Conte
Director

Accepted this 5th day of
September, 2000

/s/ George Uveges
--------------------------
George Uveges

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