Document:

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                                                                   Exhibit 10.15

                         SECURITIES PURCHASE AGREEMENT
                         -----------------------------

      SECURITIES PURCHASE AGREEMENT (this "Agreement"), dated as of January 10,
2000, by and among BIOSOURCE INTERNATIONAL, INC., a corporation organized under
the laws of the State of Delaware (the "Company"), and the purchasers (each a
"Purchaser" and collectively the "Purchasers")) set forth on the execution page
hereof (the "Execution Page").

                                 R E C I T A L S
                                 - - - - - - - -

      A. The Company and the Purchasers are executing and delivering this
Agreement in reliance upon the exemption from securities registration afforded
by the provisions of Regulation D ("Regulation D"), as promulgated by the United
States Securities and Exchange Commission (the "SEC") under the Securities Act
of 1933, as amended (the "Securities Act").

      B. The Purchasers desire to purchase subject to the terms and conditions
stated in this Agreement, (i) 371,300 shares of the Company's Series B
Convertible Preferred Stock, par value $.001 per share (the "Preferred Stock")
with the terms and conditions set forth in the Certificate of Designation of
Preferences, Rights and Limitations of Series B Preferred Stock of BioSource
International, Inc. in the form attached hereto as Exhibit A (the "Certificate
                                                   ---------
of Designations"), and (ii) warrants in the form attached hereto as Exhibit B
                                                                    ---------
(including any warrants issued in replacement thereof, the "Warrants"), to
acquire 1,287,000 shares of the Company's common stock, par value $.001 per
share (the "Common Stock"). The shares of Common Stock issuable upon conversion
of or otherwise pursuant to the Preferred Stock are referred to herein as the
"Conversion Shares." The shares of Common Stock issuable upon exercise of or
otherwise pursuant to the Warrants are referred to herein as the "Warrant
Shares."

      C. The exercise price of the Warrants shall be $7.77 per share of Common
Stock which represents a price of 112% of the market value of the Common Stock
(based on the closing price of the Common Stock of $6.9375 per share on January
7, 2000).

      D. Contemporaneous with the execution and delivery of this Agreement, the
parties hereto are executing and delivering an Investor Rights Agreement in the
form attached hereto as Exhibit C (the "Investor Rights Agreement"), pursuant to
                        ---------
which the Company has agreed to provide certain rights to the Purchasers,
including registration rights under the Securities Act and the rules and
regulations promulgated thereunder, and applicable state securities laws.

                                A G R E E M E N T
                                - - - - - - - - -

      NOW, THEREFORE, the Company and the Purchasers hereby agree as follows:

1.    CERTAIN DEFINITIONS.
      -------------------

      For purposes of this Agreement, the following terms shall have the
meanings ascribed to them as provided below:

                                       1
<PAGE>

      "Business Day" shall mean any day on which the principal United States
securities exchange or trading market on which the Common Stock is listed or
traded as reported by NTMS (as defined below) is open for trading.

      "Investment Amount" for each Purchaser shall mean the dollar amount to be
invested in the Company at the Closing pursuant to this Agreement by such
Purchaser, as set forth on the Execution Page hereto executed by such Purchaser.

      "Material Adverse Effect" shall mean any event, condition or circumstance
that has had or is reasonably likely to have a material adverse effect on (i)
the Securities, (ii) the ability of the Company to perform its obligations
hereunder (including, without limitation, the issuance of the Shares and the
Warrants), under the Certificate of Designations (including, without limitation,
the issuance of the Conversion Shares), under the Warrants (including, without
limitation, the issuance of the Warrant Shares) or under the Investor Rights
Agreement or (iii) the business, operations, properties, prospects or financial
condition of the Company and its subsidiaries, taken as a whole.

      "NTMS" shall mean the Research Service of Nasdaq Trading and Market
Services (or a comparable reporting service of national reputation if the
Research Service of Nasdaq Trading and Market Services is not then reporting
closing bid prices of such security).

      "Securities" shall mean the Shares, the Conversion Shares, the Warrants
and the Warrant Shares.

      "Shares" means the shares of Preferred Stock to be issued and sold by the
Company and purchased by the Purchasers at the Closing.

2.    PURCHASE AND SALE OF SHARES AND WARRANTS.
      ----------------------------------------

      a. Generally. Subject to the satisfaction (or waiver) of the conditions
set forth in Section 6 and Section 7 below, the Purchasers shall purchase the
number of Shares and Warrants determined as provided in this Section 2, and the
Company shall issue and sell such number of Shares and Warrants to the
Purchasers for the Investment Amount as provided below.

      b. Number of Closing Shares and Warrants; Form of Payment; Closing Date.
         --------------------------------------------------------------------
            i. On the Closing Date (as defined below), the Company shall sell
and the Purchasers shall buy (A) 371,300 shares of Preferred Stock at $24.24 per
share, and (B) Warrants to purchase 1,287,000 shares of Common Stock at an
exercise price of $7.77 per share.

            ii. On the Closing Date, each Purchaser shall pay an amount in cash
equal to its Investment Amount against delivery of certificates representing the
Shares and duly executed Warrants being purchased by such Purchaser, and the
Company shall deliver to each Purchaser certificates representing the Shares
being purchased by such Purchaser and duly executed Warrants being purchased by
such Purchaser.

                                       2
<PAGE>

            iii. Subject to the satisfaction (or waiver) of the conditions set
forth in Section 6 and Section 7 below, the closing of the transactions
contemplated by this Agreement (the "Closing") shall take place at 10:00 a.m. on
the date which is the earlier of (i) 30 days from the date hereof, or (ii) two
business days after the Purchasers have received all investment funds pursuant
to their call on investors (the "Closing Date"). The Closing shall occur at the
offices of Troop Steuber Pasich Reddick & Tobey, LLP, Los Angeles, California,
or at such other place as the Purchasers and the Company may otherwise mutually
agree.

3.    THE PURCHASER'S REPRESENTATIONS AND WARRANTIES.
      ----------------------------------------------

      The Purchasers represent and warrant to the Company as follows:

      a. Purchase for Own Account. Each Purchaser is purchasing the Shares and
         ------------------------
Warrants for its own account and not with a present view towards the
distribution thereof. Each Purchaser understands that it must bear the economic
risk of this investment indefinitely, unless the Securities are registered
pursuant to the Securities Act and any applicable state securities or blue sky
laws or an exemption from such registration is available, and that the Company
has no present intention of registering any such Securities other than as
contemplated by the Investor Rights Agreement. Notwithstanding anything in this
Section 3(a) to the contrary, by making the foregoing representation, the
Purchasers do not agree to hold the Securities for any minimum or other specific
term and reserves the right to dispose of the Securities at any time in
accordance with or pursuant to a registration statement or an exemption from
registration under the Securities Act and any applicable state securities laws.

      b. Information. The Purchasers have been furnished all materials relating
         -----------
to the business, finances and operations of the Company and its subsidiaries and
materials relating to the offer and sale of the Securities that have been
requested by them. The Purchasers have been afforded the opportunity to ask
questions of the Company and have received what they believe to be satisfactory
answers to any such inquiries. Each Purchaser understands that its investment in
the Securities involves a high degree of risk. Neither such inquiries nor any
other due diligence investigation conducted by the Purchasers or their counsel
or any of their representatives shall modify, amend or affect the Purchasers'
right to rely on the Company's representations and warranties contained in
Section 4 below.

      c. Governmental Review. Each Purchaser understands that no United States
         -------------------
federal or state agency or any other government or governmental agency has
passed upon or made any recommendation or endorsement of the Securities.

      d. Authorization; Enforcement. Each Purchaser has the requisite power and
         --------------------------
authority to enter into and perform its obligations under this Agreement and to
purchase the Shares and the Warrants in accordance with the terms hereof. This
Agreement has been duly and validly authorized, executed and delivered on behalf
of such Purchaser and is a valid and binding agreement of such Purchaser
enforceable against such Purchaser in accordance with its terms, subject to
applicable bankruptcy, insolvency, reorganization, moratorium, fraudulent
transfer and other laws affecting creditors' rights and remedies generally and
to general principles of equity (regardless of whether enforcement is sought in
a proceeding at law or in equity).

                                       3
<PAGE>

      e. Transfer or Resale. Each Purchaser understands that (i) except as
         ------------------
provided in the Investor Rights Agreement, the Securities have not been and are
not being registered under the Securities Act or any state securities laws, and
may not be transferred unless (a) subsequently registered thereunder, or (b)
such Purchaser shall have delivered to the Company an opinion of counsel
reasonably acceptable to the Company (which opinion shall be in form, substance
and scope customary for opinions of counsel in comparable transactions) to the
effect that the Securities to be sold or transferred may be sold or transferred
under an exemption from such registration, or (c) sold under Rule 144
promulgated under the Securities Act (or a successor rule), or (d) sold or
transferred to an affiliate of such Purchaser or a partner or member of such
Purchaser pursuant to an exemption under the Securities Act; and (ii) neither
the Company nor any other person is under any obligation to register such
Securities under the Securities Act or any state securities laws or to comply
with the terms and conditions of any exemption thereunder, in each case, other
than pursuant to the Investor Rights Agreement.

      f. Legends. Each Purchaser understands that the Shares and the Warrants
         -------
and, until such time as the Shares (or the Conversion Shares) and the Warrants
(or the Warrant Shares) have been registered under the Securities Act as
contemplated by the Investor Rights Agreement or otherwise may be sold by the
Purchaser under Rule 144, the certificates for the Shares, Conversion Shares,
Warrants and Warrant Shares may bear a restrictive legend in substantially the
following form:

            The securities represented by this certificate have not been
            registered under the Securities Act of 1933, as amended, or the
            securities laws of any state of the United States. The securities
            represented hereby may not be offered or sold in the absence of an
            effective registration statement for the securities under applicable
            securities laws unless offered, sold or transferred under an
            available exemption from the registration requirements of those
            laws.

      The legend set forth above shall be removed and the Company shall issue a
certificate without such legend to the holder of any Security upon which it is
stamped, if (a) the sale of such Security is registered under the Securities Act
or (b) in connection with the resale of such Security, such holder provides the
Company with an opinion of counsel, in form, substance and scope customary for
opinions of counsel in comparable transactions, or a "no action" letter from the
staff of the SEC, in either case to the effect that a public sale or transfer of
such Security may be made without registration under the Securities Act or (c)
when such Security may be sold by a person who is not an "affiliate" of the
Company under Rule 144(k). Each Purchaser agrees to sell all Securities,
including those represented by a certificate(s) from which the legend has been
removed, pursuant to an effective registration statement or under an exemption
from the registration requirements of the Securities Act. The Company
acknowledges that the Purchasers may distribute the Shares or Warrants to their
respective partners or members in accordance with the terms of their respective
organizational documents and the legend set forth above shall be removed for
Securities held by holders who can sell such Securities pursuant to Rule 144(k).

      g. Accredited Investor Status. Each Purchaser is an "accredited investor"
         --------------------------
as that term is defined in Rule 501(a) of Regulation D. Neither the Purchasers
nor any of their

                                       4
<PAGE>

respective affiliates is registered as a broker or dealer under Section 15(a) of
the Securities Exchange Act of 1934, as amended (the "Exchange Act"), affiliated
with any broker or dealer registered under Section 15(a) of the Exchange Act, or
a member of the NASD (as defined below).

4.    REPRESENTATIONS AND WARRANTIES OF THE COMPANY.
      ---------------------------------------------

      The Company represents and warrants to the Purchasers as follows:

      a. Organization and Qualification. The Company and each of its
         ------------------------------
subsidiaries is a corporation duly organized and existing under the laws of the
jurisdiction in which it is incorporated, and has the requisite corporate power
to own its properties and to carry on its business as now being conducted. The
Company and each of its subsidiaries is duly qualified as a foreign corporation
to do business and is in good standing in every jurisdiction in which the nature
of the business conducted by it makes such qualification necessary and where the
failure so to qualify would have, individually or in the aggregate, a Material
Adverse Effect. Schedule 4(a) sets forth the Company's jurisdiction of
incorporation and the name of each of the Company's subsidiaries and its
jurisdiction of incorporation. All of the outstanding equity securities of each
subsidiary is owned by the Company or one or more of its subsidiaries.

      b. Authorization; Enforcement. (i) The Company has the requisite corporate
         --------------------------
power and authority to enter into and perform its obligations under this
Agreement, the Certificate of Designations, the Warrants and the Investor Rights
Agreement, to issue and sell the Shares and the Warrants in accordance with the
terms hereof and to issue the Conversion Shares upon conversion of the Shares
and the Warrant Shares upon exercise of the Warrants in accordance with the
terms of the Certificate of Designations and Warrants, respectively; (ii) the
execution, delivery and performance of this Agreement, Certificate of
Designations, the Warrants and the Investor Rights Agreement by the Company and
the consummation by it of the transactions contemplated hereby and thereby
(including, without limitation, the reservation for issuance and issuance of the
Shares and the Conversion Shares and the issuance of the Warrants and the
reservation for issuance and issuance of the Warrant Shares) have been duly
authorized by the Company's Board of Directors and no further consent or
authorization of the Company, its Board of Directors or its shareholders is
required; (iii) this Agreement has been duly executed and delivered by the
Company; and (iv) this Agreement constitutes, and, upon execution and delivery
by the Company and the other parties thereto to the extent required of the
Investor Rights Agreement and the Warrants, such agreements will constitute,
valid and binding obligations of the Company enforceable against the Company in
accordance with their respective terms, subject to applicable bankruptcy,
insolvency, reorganization, moratorium, fraudulent transfer and other laws
affecting creditors' rights and remedies generally and to general principles of
equity (regardless of whether enforcement is sought in a proceeding at law or in
equity). The Company has delivered to the Purchasers all correspondence and
filings between the Company and/or its representatives and the NASD with respect
to this Agreement and the transactions contemplated hereby. The Board of
Directors of the Company has taken all actions necessary under the Delaware
General Corporation Law to ensure that Section 203 does not and will not apply
to the transactions contemplated hereby or any "business combination" with any
"interested stockholder" after the date hereof. On or prior to the Closing, the
Company will file the Certificate of Designations with the Secretary of State of
the State of Delaware. The Board of

                                       5
<PAGE>

Directors of the Company has taken all actions necessary under the terms of its
Shareholder Rights Plan to ensure that such Shareholder Rights Plan does not and
will not apply to the transactions contemplated hereby.

      c. Capitalization. The capitalization of the Company as of the date hereof
         --------------
is set forth on Schedule 4(c), including the authorized capital stock, the
                -------------
number of shares issued and outstanding, the number of shares issuable and
reserved for issuance pursuant to the Company's stock option plans, the number
of shares issuable and reserved for issuance pursuant to securities exercisable
for, or convertible into or exchangeable for any shares of capital stock. All of
such outstanding shares of the Company's capital stock have been, or upon
issuance will be, validly issued, fully paid and nonassessable. Except as set
forth on Schedule 4(c), no shares of capital stock of the Company (including the
         -------------
Shares, the Conversion Shares and the Warrant Shares) are subject to preemptive
rights or any other similar rights of the shareholders of the Company or any
liens or encumbrances. Except for the Securities and as disclosed in Schedule
                                                                     --------
4(c), as of the date of this Agreement, (i) there are no outstanding securities,
----
options, warrants, scrip, rights to subscribe to, calls or commitments of any
character whatsoever to which the Company is a party relating to the issuance by
the Company of securities or rights convertible into or exercisable or
exchangeable for, any shares of capital stock of the Company, or arrangements by
which the Company is or may become bound to issue additional shares of capital
stock of the Company, and (ii) there are no agreements or arrangements under
which the Company is obligated to register the sale of any of its securities
under the Securities Act (except the Investor Rights Agreement). The Company has
made available to the Purchasers through its filings with the SEC true and
correct copies of the Company's Certificate of Incorporation as in effect on the
date hereof ("Certificate of Incorporation"), the Company's By-laws as in effect
on the date hereof (the "By-laws") and all other instruments and agreements
governing securities convertible into or exercisable or exchangeable for capital
stock of the Company, except for stock options granted under any employee
benefit plan or director stock option plan of the Company.

      d. Issuance of Shares. The Shares are duly authorized and when issued and
         ------------------
paid for in accordance with the terms hereof, will be validly issued, fully paid
and non-assessable, and free from all taxes, liens, claims and encumbrances
(other than those imposed through acts or omissions of the Purchaser thereof),
and will not be subject to preemptive rights, rights of first refusal or other
similar rights of shareholders of the Company and will not impose personal
liability upon the holder thereof. The Conversion Shares are duly authorized and
reserved for issuance, and, upon conversion of the Shares in accordance with the
terms of the Certificate of Designations, will be validly issued, fully paid and
non-assessable and free from all taxes and liens, claims and encumbrances (other
than those imposed through acts or omissions of the Purchaser thereof), and will
not be subject to preemptive rights or other similar rights of shareholders of
the Company and will not impose personal liability upon the holder thereof. The
Warrant Shares are duly authorized and reserved for issuance, and, upon exercise
of the Warrants in accordance with the terms thereof, will be validly issued,
fully paid and non-assessable and free from all taxes and liens, claims and
encumbrances (other than those imposed through acts or omissions of the
Purchaser thereof), and will not be subject to preemptive rights or other
similar rights of shareholders of the Company and will not impose personal
liability upon the holder thereof.

                                       6
<PAGE>

      e. No Conflicts. The execution, delivery and performance of this
         ------------
Agreement, the Investor Rights Agreement and the Warrants by the Company, and
the consummation by the Company of the transactions contemplated hereby and
thereby (including, without limitation, the reservation for issuance and
issuance of the Shares, the Conversion Shares, the Warrant Shares and the
issuance of the Warrants) will not (i) conflict with or result in a violation of
the Certificate of Incorporation (as amended by the Certificate of Designations)
or By-laws or (ii) conflict with, or constitute a default (or an event which,
with notice or lapse of time or both, would become a default) under, or give to
others any rights of termination, amendment, acceleration or cancellation of any
agreement, indenture or instrument to which the Company is a party, or result in
a violation of any law, rule, regulation, order, judgment or decree (including
(assuming the accuracy of the representations and warranties of the Purchasers)
the United States federal and state securities laws and regulations) applicable
to the Company or by which any property or asset of the Company is bound or
affected (except, with respect to clause (ii), for such conflicts, defaults,
terminations, amendments, accelerations, cancellations and violations as would
not, individually or in the aggregate, have a Material Adverse Effect). The
Company is not in violation of its Certificate of Incorporation, By-laws and
other organizational documents and is not in default (and no event has occurred
which, with notice or lapse of time or both, would put the Company in default)
under, nor has there occurred any event giving others (with notice or lapse of
time or both) any rights of termination, amendment, acceleration or cancellation
of, any agreement, indenture or instrument to which the Company is a party,
except for actual or possible violations, defaults or rights as would not,
individually or in the aggregate, have a Material Adverse Effect. The businesses
of the Company are not being conducted in violation of any law, ordinance or
regulation of any governmental entity, except for actual or possible violations,
if any, the sanctions for which either singly or in the aggregate would not have
a Material Adverse Effect. Except as specifically contemplated by this Agreement
and as required under the Securities Act and any applicable state securities
laws, the Company is not required to obtain any consent, approval, authorization
or order of, or make any filing or registration with, any court or governmental
agency or any regulatory or self regulatory agency in order for it to execute,
deliver or perform any of its obligations under this Agreement including without
limitation the issuance and sale of the Shares and Warrants as provided hereby
and the issuance of the Conversion Shares upon conversion of the Shares, the
Warrants (including without limitation the issuance of the Warrant Shares) or
the Investor Rights Agreement, in each case in accordance with the terms hereof
or thereof. The Company is not in violation of the listing requirements of the
Nasdaq National Market ("NASDAQ") and does not reasonably anticipate that the
Common Stock will be delisted by NASDAQ in the foreseeable future based on its
rules (and interpretations thereof) as currently in effect.

      f. SEC Documents; Financial Statements. The Company has timely filed all
         -----------------------------------
reports, schedules, forms, statements and other documents required to be filed
by it with the SEC pursuant to the Exchange Act, and has filed all registration
statements and other documents required to be filed by it with the SEC pursuant
to the Securities Act (all of the foregoing filed prior to the date hereof, and
all exhibits included therein and financial statements and schedules thereto and
documents incorporated by reference therein, being hereinafter referred to
herein as the "SEC Documents"). The Company has made available to the Purchasers
true and complete copies of the SEC Documents. As of their respective dates, the
SEC Documents complied in all material respects with the requirements of the
Exchange Act or the Securities Act, as the case may be, and the rules and
regulations of the SEC promulgated thereunder applicable to the SEC

                                       7
<PAGE>

Documents, and none of the SEC Documents, at the time they were filed with the
SEC, contained any untrue statement of a material fact or omitted to state a
material fact required to be stated therein or necessary in order to make the
statements therein, in light of the circumstances under which they were made,
not misleading. Any statements made in any such SEC Documents that are or were
required to be updated or amended under applicable law have been so updated or
amended. As of their respective dates, the financial statements of the Company
included in the SEC Documents complied in all material respects with applicable
accounting requirements and the published rules and regulations of the SEC
applicable with respect thereto. Such financial statements have been prepared in
accordance with United States generally accepted accounting principles,
consistently applied, during the periods involved (except (i) as may be
otherwise indicated in such financial statements or the notes thereto, or (ii)
in the case of unaudited interim statements, to the extent they may not include
footnotes or may be condensed or summary statements) and fairly present in all
material respects the consolidated financial position of the Company as of the
dates thereof and the results of their operations and cash flows for the periods
then ended (subject, in the case of unaudited statements, to normal and
recurring year-end audit adjustments which are not material). Except as set
forth in the SEC Documents, the Company has no liabilities, contingent or
otherwise, other than (i) liabilities incurred in the ordinary course of
business subsequent to the date of such SEC Documents and (ii) obligations under
contracts and commitments incurred in the ordinary course of business and not
required under generally accepted accounting principles to be reflected in such
SEC Documents, which liabilities and obligations referred to in clauses (i) and
(ii), individually or in the aggregate, would not have a Material Adverse
Effect.

      g. Absence of Certain Changes. Except as disclosed in the SEC Documents,
         --------------------------
there has been no change or development which individually or in the aggregate
has had or could have a Material Adverse Effect.

      h. Absence of Litigation. Except as disclosed in the SEC Documents, there
         ---------------------
is no action, suit, proceeding, inquiry or investigation before or by any court,
public board, government agency, self-regulatory organization or body pending
or, to the knowledge of the Company, threatened against or affecting the
Company, or any of their directors or officers in their capacities as such which
would have, individually or in the aggregate, a Material Adverse Effect.

      i. Acknowledgment Regarding the Purchaser's Purchase of the Securities.
         -------------------------------------------------------------------
The Company acknowledges and agrees that the Purchasers are not acting as
financial advisors or, acting as fiduciaries of the Company (or in any similar
capacity) with respect to this Agreement or the transactions contemplated
hereby, and the relationship between the Company and the Purchasers is "arms
length" and that any statement made by the Purchasers or any of their respective
representatives or agents in connection with this Agreement and the transactions
contemplated hereby is not advice or a recommendation and is merely incidental
to the Purchasers' purchases of Securities and has not been relied upon by the
Company, its officers or directors in any way. The Company further represents to
the Purchasers that the Company's decision to enter into this Agreement has been
based solely on an independent evaluation by the Company and its
representatives.

                                       8
<PAGE>

      j. No Brokers. The Company has not engaged any person to which or to whom
         ----------
brokerage commissions, finder's fees, financial advisory fees or similar
payments are or will become due in connection with this Agreement or the
transactions contemplated hereby.

      k. Form S-3 Eligibility. The Company is currently eligible to register the
         --------------------
resale of its Common Stock on a registration statement on Form S-3 under the
Securities Act. There exist no facts or circumstances (including without
limitation any required approvals or waivers of any circumstances that may delay
or prevent the obtaining of accountant's consents) that would prohibit or delay
the preparation and filing of a registration statement on Form S-3 with respect
to the Registrable Securities (as defined in the Investor Rights Agreement).

      l. Intellectual Property. The Company owns or possesses sufficient legal
         ---------------------
rights to all patents, trademarks, service marks, trade names, copyrights, trade
secrets, licenses, information and proprietary rights and processes necessary
for its business without any conflict with, or infringement of, the rights of
others. The Company has not received any communications alleging that the
Company has violated or, by conducting its business, would violate any of the
patents, trademarks, service marks, trade names, copyrights, trade secrets or
other proprietary rights or processes of any other person or entity. The Company
is not aware that any of its employees is obligated under any contract
(including licenses, covenants or commitments of any nature) or other agreement,
or subject to any judgment, decree or order of any court or administrative
agency, that would interfere with the use of such employee's best efforts to
promote the interest of the Company or that would conflict with the Company's
business. Neither the execution or delivery of this Agreement, nor the carrying
on of the Company's business by the employees of the Company, nor the conduct of
the Company's business as proposed, will, to the Company's knowledge, conflict
with or result in a breach of the terms, conditions, or provisions of, or
constitute a default under, any contract, covenant or instrument under which any
such employee is now obligated. The Company does not believe it is or will be
necessary to use any inventions of any of its employees (or persons it currently
intends to hire) made prior to their employment by the Company. Set forth in
Schedule 4(l)_ is a listing of all patents, trademarks and licenses of the
Company.

      m. Taxes. The Company has filed all tax returns and reports as required by
         -----
law. These returns and reports are true and correct in all material respects.
The Company has paid all taxes and other assessments due.

      n. Environmental Laws. The Company is not in violation of any applicable
         ------------------
statute, law or regulation relating to the environment or occupational health
and safety, and to its knowledge, no material expenditures are or will be
required in order to comply with any such existing statute, law or regulation.
No Hazardous Materials (as defined below) are used or have been used, stored, or
disposed of by the Company or, to the Company's knowledge, by any other person
or entity on any property owned, leased or used by the Company. For the purposes
of the preceding sentence, "Hazardous Materials" shall mean (a) materials which
are listed or otherwise defined as "hazardous" or "toxic" under any applicable
local, state, federal and/or foreign laws and regulations that govern the
existence and/or remedy of contamination on property, the protection of the
environment from contamination, the control of hazardous wastes, or other
activities involving hazardous substances, including building materials or (b)
any petroleum products or nuclear materials.

                                       9
<PAGE>

      o. ERISA Compliance. The Company is in compliance in all material respects
         ----------------
with all presently applicable provisions of the Employee Retirement Income
Security Act of 1974, as amended, including the regulations and published
interpretations thereunder ("ERISA"). The Company does not have and has never
had any "defined benefit plan" as defined under ERISA and is not and has never
been obligated to contribute to a "multi-employer plan" as defined in ERISA.

      p. No Misrepresentations. No representation, warranty, acknowledgement or
         ---------------------
certification of the Company contained in this Agreement, any schedule, annex or
exhibit hereto or any certificate furnished by the Company to the Purchasers
pursuant to this Agreement, contains any untrue statement of a material fact or
omits to state a material fact required to be stated therein or necessary to
make the statements therein not misleading.

5.    COVENANTS.
      ---------

      a. Satisfaction of Conditions. The parties shall use reasonable best
         --------------------------
efforts to satisfy in a timely manner each of the conditions set forth in
Section 6 and Section 7 of this Agreement.

      b. Form D; Blue Sky Laws. The Company agrees to file a Form D with respect
         ---------------------
to the Securities as required under Regulation D and to provide a copy thereof
to the Purchasers promptly after such filing. The Company shall, on or prior to
the Closing Date, take such action as the Company shall reasonably determine is
necessary to qualify the Securities for sale to the Purchasers pursuant to this
Agreement under applicable securities or "blue sky" laws of the states of the
United States or obtain exemption therefrom.

      c. Reporting Status. So long as the Purchasers beneficially own any
         ----------------
Securities or have the right to acquire any Securities pursuant to this
Agreement, the Company shall timely file all reports required to be filed with
the SEC pursuant to the Exchange Act, and shall not terminate its status as an
issuer required to file reports under the Exchange Act even if the Exchange Act
or the rules and regulations thereunder would permit such termination.

      d. Use of Proceeds. The Company shall not use the net proceeds from the
         ---------------
sale of the Shares and the Warrants to repurchase any outstanding equity
securities of the Company.

      e. Upfront Fee; Expenses. At the Closing, the Company shall pay to Genstar
         ---------------------
Capital LLC a sum equal to three percent (3%) of the aggregate Investment Amount
and reimburse each Purchaser for the out-of-pocket expenses reasonably incurred
by it and its affiliates and advisors in connection with the negotiation,
preparation, execution and delivery of this Agreement, the Investor Rights
Agreement, the Warrants and the other agreements to be executed in connection
herewith, including, without limitation, its reasonable attorneys' fees and
expenses (the "Expenses").

      f. Financial Information. So long as the Purchasers hold not less than 20%
         ---------------------
of the aggregate amount of Shares and Warrants being purchased pursuant to this
Agreement, following the Closing, the Company agrees to send to the Purchasers
within ten days after the filing with the SEC, to the extent not available
through the SEC's EDGAR system, a copy of its

                                       10
<PAGE>

Annual Report on Form 10-K, its Quarterly Reports on Form 10-Q, its proxy and
information statements and any Current Reports on Form 8-K and to send to the
Purchasers at such time as they are generally available to management month end
summary financial statements, but in no event later than 30 days after the end
of each month.

      g. Reservation of Shares. The Company has and shall at all times have
         ---------------------
authorized and reserved for the purpose of issuance a sufficient number of
shares of Common Stock to provide for the issuance of the Shares as provided in
Section 2 hereof, the full conversion of the Shares and the issuance of the
Conversion Shares, and the full exercise of the Warrants and the issuance of the
Warrant Shares in connection therewith and as otherwise required hereby and by
the Warrants. The Company shall not reduce the number of shares of Common Stock
reserved for issuance under this Agreement (except as a result of the issuance
of the Conversion Shares upon conversion of the Shares), the Warrants (except as
a result of the issuance of the Warrant Shares upon the exercise of the
Warrants) or the Investor Rights Agreement, without the consent of the
Purchaser, such consent not to be unreasonably withheld, conditioned or delayed.

      h. Listing. On the Closing Date, the Company shall have applied for the
         -------
listing of the Conversion Shares and Warrant Shares, in each case, upon each
national securities exchange or automated quotation system, if any, upon which
shares of Common Stock are then listed or quoted and shall maintain, so long as
any other shares of Common Stock shall be so listed, such listing of all
Conversion Shares from time to time issuable upon conversion of the Shares and
all Warrant Shares from time to time issuable upon exercise of the Warrants. The
Company shall use all commercially reasonable efforts to continue the listing
and trading of its Common Stock on NASDAQ and will comply in all respects with
the Company's reporting, filing and other obligations under the bylaws or rules
of NASDAQ and the NASD or any other exchanges, as applicable. The Company shall
use all commercially reasonable efforts to obtain from the NASD a letter
reasonably satisfactory to the Purchasers confirming that no shareholder
approval of the transactions contemplated hereby is required under the rules of
the NASD. The Company shall deliver to the Purchasers all correspondence and
filings between the Company and/or its representatives and the NASD with respect
to this Agreement and the transactions contemplated hereby.

      i. Additional Equity Capital. The Company agrees that during the period
         -------------------------
beginning on the date hereof and ending on the date which is 90 days following
the Closing Date (the "Lock-Up Period"), the Company will not, without the prior
written consent of the Purchasers or their designees, such consent not to be
unreasonably withheld, conditioned or delayed, contract with any party to obtain
additional financing in which any equity or equity-linked securities are issued
(including any debt financing with an equity component) (an "Equity Financing")
pursuant to any offering exempt from the registration requirements of the
Securities Act which grants any registration rights exercisable within six
months of the Closing Date. The limitations referred to in this Section 5(i)
shall not apply to (i) any transaction involving issuances of securities as
consideration in a merger, consolidation or acquisition of assets, or in
connection with any strategic partnership, collaboration or joint venture (the
primary purpose of which is not to raise equity capital), or as consideration
for the acquisition of a business, product or license by the Company, (ii) the
issuance of securities pursuant to an underwritten public offering, (iii) the
issuance of securities upon exercise or conversion of the Company's options,
warrants or other convertible securities outstanding as of the date hereof as
set forth in Schedule
             --------

                                       11
<PAGE>

4(c) or (iv) the grant of additional options or warrants, or the issuance of
---
additional securities, under any duly authorized Company stock option, stock
purchase or restricted stock plan for the benefit of the Company's employees,
consultants or directors.

      j. Operations in the Ordinary Course. Between the date of this Agreement
         ---------------------------------

and the Closing Date, the Company shall, and shall cause its subsidiaries to,
conduct its business in all material respects in the ordinary course consistent
with past practice, and shall use reasonable best efforts to preserve intact its
business organization and maintain its existing relations and goodwill with
customers, suppliers, regulators, distributors, creditors, lessors and others
having business dealings with it.

6.    CONDITIONS TO THE COMPANY'S OBLIGATION TO SELL.
      ----------------------------------------------

      The obligation of the Company hereunder to issue and sell Shares and
Warrants to the Purchasers at the Closing hereunder is subject to the
satisfaction, at or before the Closing Date, of each of the following conditions
thereto; provided, however, that these conditions are for the Company's sole
benefit and may be waived by the Company at any time in its sole discretion.

      a. The representations and warranties of the Purchasers shall be true and
correct as of the date when made and as of the Closing Date as though made at
that time (except for representations and warranties that speak as of a specific
date, which representations and warranties shall be true and correct as of such
date), and the Purchasers shall have performed, satisfied and complied in all
material respects with the covenants, agreements and conditions required by this
Agreement to be performed, satisfied or complied with by the Purchasers at or
prior to the Closing Date.

      b. No statute, rule, regulation, executive order, decree, ruling,
injunction, action, proceeding or interpretation shall have been enacted,
entered, promulgated, endorsed or adopted by any court or governmental authority
of competent jurisdiction or any self-regulatory organization, or the staff of
any thereof, having authority over the matters contemplated hereby which
questions the validity of, or challenges or prohibits the consummation of, any
of the transactions contemplated by this Agreement.

7.    CONDITIONS TO PURCHASERS' OBLIGATION TO PURCHASE SHARES AND WARRANTS.
      --------------------------------------------------------------------

      The obligation of the Purchasers hereunder to purchase Shares and Warrants
to be purchased by them hereunder is subject to the satisfaction, at or before
the Closing Date, of each of the following conditions; provided, however that
these conditions are for the Purchasers' sole benefit and may be waived by the
Purchasers at any time in the Purchasers' sole discretion:

      a. The Company shall have delivered to the Purchasers duly executed
certificates representing the number of Shares and duly executed Warrants as
provided in Section 2(b) above.

      b. The representations and warranties of the Company shall be true and
correct as of the date when made and as of the Closing Date as though made at
that time (except for

                                       12
<PAGE>

representations and warranties that speak as of a specific date, which
representations and warranties shall be true and correct as of such date) and
the Company shall have performed, satisfied and complied in all material
respects with the covenants, agreements and conditions required by this
Agreement to be performed, satisfied or complied with by the Company at or prior
to the Closing Date. The Purchasers shall have received a certificate, executed
on behalf of the Company by its Chief Financial Officer, dated as of the Closing
Date, to the foregoing effect and attaching true and correct copies of the
resolutions adopted by the Company's Board of Directors authorizing the
execution, delivery and performance by the Company of its obligations under this
Agreement, the Certificate of Designations, the Warrants and the Investor Rights
Agreement.

      c. No statute, rule, regulation, executive order, decree, ruling,
injunction, action, proceeding or interpretation shall have been enacted,
entered, promulgated, endorsed or adopted by any court or governmental authority
of competent jurisdiction or any self-regulatory organization, or the staff of
any thereof, having authority over the matters contemplated hereby which
questions the validity of, or challenges or prohibits the consummation of, any
of the transactions contemplated by this Agreement.

      d. The Purchasers shall have received an opinion of the Company's counsel,
dated as of the Closing Date, in the form of Exhibit D attached hereto.

      e. From the date of this Agreement through the Closing Date, there shall
not have occurred any Material Adverse Effect.

8.    GOVERNING LAW; MISCELLANEOUS.
      ----------------------------

      a. Governing Law; Jurisdiction. This Agreement shall be governed by and
         ---------------------------
construed in accordance with the laws of the State of California applicable to
contracts made and to be performed in the State of California. Each of the
parties irrevocably consents to the jurisdiction of the United States federal
courts and the state courts located in the State of California in any suit or
proceeding based on or arising under this Agreement and irrevocably agrees that
all claims in respect of such suit or proceeding may be determined in such
courts. Each of the parties irrevocably waives the defense of an inconvenient
forum to the maintenance of such suit or proceeding. Each of the parties further
agrees that service of process upon such party mailed by first class mail to the
address set forth in Section 8(f) shall be deemed in every respect effective
service of process upon such party in any such suit or proceeding. Nothing
herein shall affect the right of the Purchasers to serve process in any other
manner permitted by law. Each of the parties, agrees that a final non-appealable
judgment in any such suit or proceeding shall be conclusive and may be enforced
in other jurisdictions by suit on such judgment or in any other lawful manner.

      b. Counterparts. This Agreement may be executed in two or more
         ------------
counterparts, all of which shall be considered one and the same agreement and
shall become effective when counterparts have been signed by each party and
delivered to the other party. This Agreement, once executed by a party, may be
delivered to the other parties hereto by facsimile transmission of a copy of
this Agreement bearing the signature of the party so delivering this Agreement.
In the event any signature is delivered by facsimile transmission, the party
using such means of

                                       13
<PAGE>

delivery shall cause the manually executed Execution Page(s) hereof to be
physically delivered to the other party within five (5) days of the execution
hereof.

      c. Headings. The headings of this Agreement are for convenience of
         --------
reference and shall not form part of, or affect the interpretation of, this
Agreement.

      d. Severability. If any provision of this Agreement shall be invalid or
         ------------
unenforceable in any jurisdiction, such invalidity or unenforceability shall not
affect the validity or enforceability of the remainder of this Agreement or the
validity or enforceability of this Agreement in any other jurisdiction.

      e. Entire Agreement; Amendments; Waiver. This Agreement and the
         ------------------------------------
instruments referenced herein contain the entire understanding of the parties
with respect to the matters covered herein and therein and, except as
specifically set forth herein or therein, neither the Company nor the Purchasers
make any representation, warranty, covenant or undertaking with respect to such
matters. No provision of this Agreement may be waived or amended other than by
an instrument in writing. Any waiver by the Purchasers, on the one hand, or the
Company, on the other hand, of a breach of any provision of this Agreement shall
not operate as or be construed to be a waiver of any other breach of such
provision of or any breach of any other provision of this Agreement. The failure
of the Purchasers, on the one hand, or the Company, on the other hand to insist
upon strict adherence to any term of this Agreement on one or more occasions
shall not be considered a waiver or deprive that party of the right thereafter
to insist upon strict adherence to that term or any other term of this
Agreement.

      f. Notices. Any notices required or permitted to be given under the terms
         -------
of this Agreement shall be sent by certified or registered mail (return receipt
requested) or delivered personally or by courier or by confirmed telecopy, and
shall be effective five days after being placed in the mail, if mailed, or upon
receipt or refusal of receipt, if delivered personally or by courier or
confirmed telecopy, in each case addressed to a party. The addresses for such
communications shall be:

            If to the Company:

                  BioSource International, Inc.
                  820 Flynn Road
                  Camarillo, California  93012
                  Facsimile No.: (805) 383-5382
                  Attention:  James Chamberlain, President and Chief Executive
                              Officer

            With a copy, which shall not constitute notice, to:

                  Troop Steuber Pasich Reddick & Tobey, LLP
                  2029 Century Park East, 24th Floor
                  Los Angeles, CA  90067
                  Facsimile No.:  (310) 728-2222
                  Attention:  Scott W. Alderton, Esq.

                                       14
<PAGE>

If to a Purchaser, to the address set forth under such Purchaser's name on the
Execution Page hereto executed by such Purchaser, with a copy, which shall not
constitute notice, to:

                  Latham & Watkins
                  505 Montgomery Street, Suite 1900
                  San Francisco, CA 94111-2562
                  Facsimile No.:  (415) 395-8095
                  Attention:  Scott Haber, Esq.

      Each party hereto may from time to time change its address or facsimile
number for notices under this Section 8 by giving at least ten (10) days' prior
written notice of such changed address or facsimile number, in the case of the
Purchasers to the Company, and in the case of the Company to the Purchasers.

      g. Successors and Assigns. This Agreement shall be binding upon and inure
         ----------------------
to the benefit of the parties and their successors and assigns. The Company
shall not assign this Agreement or any rights or obligations hereunder without
the prior written consent of the Purchasers, such consent not to be unreasonably
withheld, conditioned or delayed. The Purchasers shall not assign this Agreement
or any rights or obligations hereunder without the prior written consent of the
Company, such consent not to be unreasonably withheld, conditioned or delayed,
except to an affiliate of such Purchaser, and provided further, that any such
assignee shall agree in writing with the Company to be bound by the terms and
conditions hereof and of the Investor Rights Agreement.

      h. Third Party Beneficiaries. This Agreement is intended for the benefit
         -------------------------
of the parties hereto and their respective permitted successors and assigns, and
is not for the benefit of, nor may any provision hereof be enforced by any other
person.

      i. Survival. The representations and warranties of the Company shall
         --------
survive for one (1) year following the Closing Date (other than Section 4(d)
which shall survive indefinitely), notwithstanding any due diligence
investigation conducted by or on behalf of the Purchasers. Moreover, none of the
representations and warranties made by the Company herein shall act as a waiver
of any rights or remedies the Purchasers may have under applicable federal or
state securities laws. The Company agrees to indemnify and hold harmless the
Purchasers and each of the Purchasers' respective officers, directors,
employees, partners, members, agents and affiliates for loss or damage relating
to the Securities purchased hereunder arising as a result of or related to any
breach by the Company of any of its representations or covenants set forth
herein, including advancement of expenses as they are incurred.

      j. Publicity. The Company shall have the right to approve the issuance of
         ---------
any press releases or any other public statements with respect to the
transactions contemplated hereby. Within five days after the Closing Date, the
Company shall file a Current Report on Form 8-K or other appropriate form with
the SEC disclosing the transactions contemplated hereby. Nothing contained in
this Section 8(j) shall prevent the Company from satisfying its disclosure
obligations under the federal or state securities laws or pursuant to any rule
of any trading market that is a principal trading market for any of the
Company's securities.

                                       15
<PAGE>

      k. Further Assurances. Each party shall do and perform, or cause to be
         ------------------
done and performed, all such further acts and things, and shall execute and
deliver all such other agreements, certificates, instruments and documents, as
the other party may reasonably request in order to carry out the intent and
accomplish the purposes of this Agreement and the consummation of the
transactions contemplated hereby.

      l. Termination. In the event that the Closing shall not have occurred on
         -----------
or before March 31, 2000, or such other date as the Company and the Purchasers
may mutually agree, this Agreement shall terminate at the close of business on
such date. Notwithstanding any termination of this Agreement, any party not in
breach of this Agreement shall preserve all rights and remedies it may have
against another party hereto for a breach of this Agreement prior to or relating
to the termination hereof.

      l. California Corporate Securities Law. THE SALE OF THE SECURITIES THAT
         -----------------------------------
ARE THE SUBJECT OF THIS AGREEMENT HAS NOT BEEN QUALIFIED WITH THE COMMISSIONER
OF CORPORATIONS OF THE STATE OF CALIFORNIA AND THE ISSUANCE OF SUCH SECURITIES
OR THE PAYMENT OR RECEIPT OF ANY PART OF THE CONSIDERATION FOR SUCH SECURITIES
PRIOR TO SUCH QUALIFICATION IS UNLAWFUL, UNLESS THE SALE OF SECURITIES IS EXEMPT
FROM QUALIFICATION BY SECTION 25100, 25102 OR 25105 OF THE CALIFORNIA
CORPORATIONS CODE. THE RIGHTS OF ALL PARTIES TO THIS AGREEMENT ARE EXPRESSLY
CONDITIONED UPON SUCH QUALIFICATION BEING OBTAINED, UNLESS THE SALE IS EXEMPT.

                                       16
<PAGE>

      IN WITNESS WHEREOF, the undersigned Purchaser and the Company have caused
this Agreement to be duly executed as of the date first above written.

                                    COMPANY:

                                    BIOSOURCE INTERNATIONAL, INC.

                                    By: /s/ James Chamberlain
                                       ---------------------------------
                                       James Chamberlain
                                       President and Chief Executive Officer

                                    The Purchasers:

                                    GENSTAR CAPITAL PARTNERS II, L.P.

                                    By: Genstar Capital LLC
                                        Its General Partner

                                    By: /s/ Jean-Pierre L. Conte
                                       ---------------------------------
                                        Jean-Pierre L. Conte
                                        Managing Director

                                    Investment Amount: $8,829,270.00
                                    Shares of Series B Preferred: 364,244
                                    No. of Warrants: 1,262,542

                                    Address: 555 California Street, Suite 4850
                                    San Francisco, CA  94104
                                    Telecopy No.:  (415) 834-2383
                                    Attention:  Jean-Pierre L. Conte

                                    STARGEN II LLC

                                    By: /s/ Jean-Pierre L. Conte
                                        --------------------------------
                                        Jean-Pierre L. Conte
                                        Managing Director

                                    Investment Amount: $171,042.00
                                    Shares of Series B Preferred: 7,056
                                    No. of Warrants: 24,458

                                    Address: 555 California Street, Suite 4850
                                    San Francisco, CA  94104
                                    Telecopy No.:     (415) 834-2383

                                       17
<PAGE>

                                    Attention: Jean-Pierre L. Conte

                                       18<PAGE>

                                                                   EXHIBIT 10.20

                            BIOSOURCE INTERNATIONAL
                                 820 Flynn Road
                              Camarillo, CA 93012

August 2, 2000

PRIVATE AND CONFIDENTIAL

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

Dear Russ:

This letter extends an offer to you to become the President and Chief Executive
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., the Board believes 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 President and Chief Executive Officer of the Company and as
Chairman of the Board of Directors of the Company. As the Chief Executive
Officer, you will report to the Board of Directors on executive matters and will
be responsible for 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. In those capacities, you will be principally
responsible for the growth in revenues and profits of the Company as well as
strategic direction and other executive matters.

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

Salary
------

The base salary of your position will be three hundred and ninety-five thousand
dollars ($395,000) 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.

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
<PAGE>

Mr. Russell D. Hays
August 2, 2000
Page 2

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 65% of salary.

      6)    The bonus for each individual will be split between corporate
            performance and personal objectives. This split will be determined,
            for all participants in the plan, after discussion with you as the
            Chief Executive.

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 2, 2000 was 18 7/8 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. As part
of this agreement and subject to your investment in BioSource, Genstar Capital
Partners II, L.P. and its affiliates agree to invest up to $4.5 million at the
same price at the same time as you make your investment. All shareholders will
be subject to future dilution for such things as additional grants under the
stock option plan. Both your investment and Genstar's investment are 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 350,000 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. Russell D. Hays
August 2, 2000
Page 3

Should there be a significant change whereby a Company or investor unaffiliated
with Genstar Capital LLC ("Genstar") purchases 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.

Russ, 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.
<PAGE>

Mr. Russell D. Hays
August 2, 2000
Page 4

We have extended this offer to you to become the President and Chief Executive
Officer 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 9 day of August, 2000       Accepted this 9th day of AUGUST, 2000

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

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