<PAGE>

                                                                     EXHIBIT 4.2

THE WARRANT  EVIDENCED  OR  CONSTITUTED  HEREBY,  AND ALL SHARES OF COMMON STOCK
ISSUABLE HEREUNDER,  HAVE BEEN AND WILL BE ISSUED WITHOUT REGISTRATION UNDER THE
SECURITIES ACT OF 1933, AS AMENDED (THE  "SECURITIES  ACT") AND MAY NOT BE SOLD,
OFFERED FOR SALE,  TRANSFERRED,  PLEDGED OR  HYPOTHECATED  WITHOUT  REGISTRATION
UNDER THE ACT UNLESS  EITHER (i) THE COMPANY HAS RECEIVED AN OPINION OF COUNSEL,
IN FORM AND SUBSTANCE REASONABLY SATISFACTORY TO THE COMPANY, TO THE EFFECT THAT
REGISTRATION  IS NOT REQUIRED IN CONNECTION  WITH SUCH  DISPOSITION  OR (ii) THE
SALE OF SUCH SECURITIES IS MADE PURSUANT TO SEC RULE 144.

                        WARRANT TO PURCHASE COMMON STOCK
                                       OF
                          BIOSOURCE INTERNATIONAL, INC.

NO.                                                            February __, 2000

THIS  CERTIFIES  THAT, for value  received by BioSource  International,  Inc., a
Delaware  corporation (the  "Company"),  GenStar Capital Partners II, LP, or its
                             -------
permitted registered assigns ("Holder"),  is entitled,  subject to the terms and
                               ------
conditions of this Warrant,  at any time or from time to time after the issuance
date of this Warrant (the "Effective  Date"),  and before 5:00 p.m. Pacific Time
                           ---------------
on the fifth (5th) anniversary of the Effective Date (the "Expiration Date"), to
                                                           ---------------
purchase from the Company 1,262,542 shares of Common Stock of the Company,  at a
price per share of $7.77 (the  "Purchase  Price").  Both the number of shares of
                                ---------------
Common Stock  purchasable  upon exercise of this Warrant and the Purchase  Price
are subject to adjustment and change as provided herein.

     1. CERTAIN DEFINITIONS. As used in this Warrant the following terms shall
have the following respective meanings:

        1.1 "Fair Market Value" of a share of Common Stock as of a particular
             -----------------
date shall mean:

            (a) If traded on a securities exchange or the Nasdaq National
Market, the Fair Market Value shall be deemed to be the average of the closing
prices of the Common Stock of the Company on such exchange or market over the
five (5) trading days ending immediately prior to the applicable date of
valuation;

            (b) If actively traded over-the-counter, the Fair Market Value shall
be deemed to be the average of the closing bid prices over the thirty (30)-day
period ending immediately prior to the applicable date of valuation; and

            (c) If there is no active public market, the Fair Market Value shall
be the value thereof, as agreed upon by the Company and the Holder; provided,
however, that if the Company and the Holder cannot agree on such value, such
value shall be determined by an independent valuation firm experienced in
valuing businesses such as the Company and jointly selected in good faith by the
Company and the Holder. Fees and expenses of the valuation firm shall be paid
for by the Company.
<PAGE>

        1.2 "Registered Holder" shall mean any Holder in whose name this Warrant
             -----------------
is registered upon the books and records maintained by the Company.

        1.3 "Warrant" as used herein, shall include this Warrant and any warrant
             -------
delivered in substitution or exchange therefor as provided herein.

        1.4 "Common Stock" shall mean the Common Stock of the Company and any
             ------------
other securities at any time receivable or issuable upon exercise of this
Warrant.

     2. EXERCISE OF WARRANT.

        2.1 Payment. Subject to compliance with the terms and conditions of this
            -------
Warrant and applicable securities laws, this Warrant may be exercised, in whole
or in part at any time or from time to time, on or before the Expiration Date by
the delivery (including, without limitation, delivery by facsimile) of the form
of Notice of Exercise attached hereto as Exhibit A (the "Notice of Exercise"),
duly executed by the Holder, at the principal office of the Company, and as soon
as practicable after such date, surrendering

            (a) this Warrant at the principal office of the Company, and

            (b) payment, (i) in cash (by check) or by wire transfer, (ii) by
cancellation by the Holder of indebtedness of the Company to the Holder; or
(iii) by a combination of (i) and (ii), of an amount equal to the product
obtained by multiplying the number of shares of Common Stock being purchased
upon such exercise by the then effective Purchase Price (the "Exercise Amount").
                                                              ---------------

        2.2 Net Issue Exercise. In lieu of the payment methods set forth in
            ------------------
Section 2.1(b) above, the Holder may elect to exchange all or some of this
--------------
Warrant for shares of Common Stock equal to the value of the amount of the
Warrant being exchanged on the date of exchange. If Holder elects to exchange
this Warrant as provided in this Section 2.2, Holder shall tender to the Company
                                 -----------
the Warrant for the amount being exchanged, along with written notice of
Holder's election to exchange some or all of the Warrant, and the Company shall
issue to Holder the number of shares of the Common Stock computed using the
following formula:

                           X =    Y (A-B)
                                -------------
                                     A

                       Where:   X =  the number of shares of Common Stock to be
                                     issued to Holder.

                                Y =  the  number  of  shares of Common  Stock
                                     purchasable  under the  amount of the
                                     Warrant being exchanged (as adjusted to the
                                     date of such  calculation).

                                A =  the Fair Market Value of one share of the
                                     Common Stock.

                                B =  Purchase Price (as adjusted to the date of
                                     such calculation).

        2.3 "Easy Sale" Exercise. In lieu of the payment methods set forth in
             -------------------
Sections 2.1(b) and 2.2, above, when permitted by law and applicable regulations
-----------------------
(including Nasdaq and NASD rules), the Holder may pay the Purchase Price through
a "same day sale" commitment from the Holder (and if applicable a broker-dealer
that is a member of the National Association of Securities Dealers (an "NASD
                                                                        ----
Dealer")), whereby the Holder irrevocably elects to exercise this
------

                                       2
<PAGE>

Warrant and to sell a portion of the shares so purchased to pay the Purchase
Price and the Holder (or, if applicable, the NASD Dealer) commits upon sale (or,
in the case of the NASD Dealer, upon receipt) of such shares to forward the
Purchase Price directly to the Company.

        2.4 Stock Certificates; Fractional Shares. As soon as practicable on or
            -------------------------------------
after the date of any exercise of this Warrant, the Company shall issue and
deliver to the person or persons entitled to receive the same a certificate or
certificates for the number of whole shares of Common Stock issuable upon such
exercise, together with cash in lieu of any fraction of a share equal to such
fraction of the current Fair Market Value of one whole share of Common Stock as
of such date of exercise. No fractional shares or scrip representing fractional
shares shall be issued upon an exercise of this Warrant.

        2.5 Partial Exercise; Effective Date of Exercise. In case of any partial
            --------------------------------------------
exercise of this Warrant, the Company shall cancel this Warrant upon surrender
hereof and shall execute and deliver a new Warrant of like tenor and date for
the balance of the shares of Common Stock purchasable hereunder. This Warrant
shall be deemed to have been exercised immediately prior to the close of
business on the date of its surrender for exercise as provided above. The person
entitled to receive the shares of Common Stock issuable upon exercise of this
Warrant shall be treated for all purposes as the holder of record of such shares
as of the close of business on the date the Holder is deemed to have exercised
this Warrant.

        2.6 Vesting. This Warrant shall vest fully upon issuance.
            -------

     3. VALID ISSUANCE: TAXES. All shares of Common Stock issued upon the
exercise of this Warrant shall be validly issued, fully paid and non-assessable,
and the Company shall pay all taxes and other governmental charges that may be
imposed in respect of the issue or delivery thereof. The Company shall not be
required to pay any tax or other charge imposed in connection with any transfer
involved in the issuance of any certificate for shares of Common Stock in any
name other than that of the Registered Holder of this Warrant, and in such case
the Company shall not be required to issue or deliver any stock certificate or
security until such tax or other charge has been paid, or it has been
established to the Company's reasonable satisfaction that no tax or other charge
is due.

     4. ADJUSTMENT OF PURCHASE PRICE AND NUMBER OF SHARES. The number of
shares of Common Stock issuable upon exercise of this Warrant (or any shares of
stock or other securities or property receivable or issuable upon exercise of
this Warrant) and the Purchase Price are subject to adjustment upon occurrence
of the following events:

        4.1 Adjustment for Stock Splits, Stock Subdivisions or Combinations of
            ------------------------------------------------------------------
Shares. The Purchase Price of this Warrant shall be proportionally decreased and
------
the number of shares of Common Stock issuable upon exercise of this Warrant (or
any shares of stock or other securities at the time issuable upon exercise of
this Warrant) shall be proportionally increased to reflect any stock split or
subdivision of the Company's Common Stock. The Purchase Price of this Warrant
shall be proportionally increased and the number of shares of Common Stock
issuable upon exercise of this Warrant (or any shares of stock or other
securities at the time issuable upon exercise of this Warrant) shall be
proportionally decreased to reflect any combination of the Company's Common
Stock.

        4.2 Adjustment for Dividends or Distributions of Stock or Other
            -----------------------------------------------------------
Securities or Property. In case the Company shall make or issue, or shall fix a
----------------------
record date for the determination

                                       3
<PAGE>

of eligible holders entitled to receive, a dividend or other distribution with
respect to the Common Stock (or any shares of stock or other securities at the
time issuable upon exercise of the Warrant) payable in (a) securities of the
Company or (b) assets or (c) cash dividends paid or payable solely out of
retained earnings, but only if such cash dividends are Extraordinary Dividends
(as defined below), then, in each such case, the Holder of this Warrant on
exercise hereof at any time after the consummation, effective date or record
date of such dividend or other distribution, shall receive, in addition to the
shares of Common Stock (or such other stock or securities) issuable on such
exercise prior to such date, and without the payment of additional consideration
therefor, the securities, assets or cash to which such Holder would have been
entitled upon such date if such Holder had exercised this Warrant on the date
hereof and had thereafter, during the period from the date hereof to and
including the date of such exercise, retained such shares and all such
additional securities, assets or cash distributed with respect to such shares as
aforesaid during such period giving effect to all adjustments called for by this
Section 4. "Extraordinary Dividends" shall mean dividends or distributions
---------
declared with respect to the Common Stock that are in an amount greater than 3%
of the aggregate Fair Market Value of the shares of capital stock receiving such
dividends as of the trading day prior to the declaration of such dividends or
distributions.

        4.3 Reclassification. If the Company, by reclassification of securities
            ----------------
or otherwise, shall change any of the securities as to which purchase rights
under this Warrant exist into the same or a different number of securities of
any other class or classes, this Warrant shall thereafter represent the right to
acquire such number and kind of securities as would have been issuable as the
result of such change with respect to the securities that were subject to the
purchase rights under this Warrant immediately prior to such reclassification or
other change, and the Purchase Price therefor shall be appropriately adjusted,
all subject to further adjustment as provided in this Section 4. No adjustment
shall be made pursuant to this Section 4.3 upon any conversion or redemption of
                               -----------
the Common Stock which is the subject of Section 4.5.
                                         -----------

        4.4 Adjustment for Capital Reorganization, Merger or Consolidation. In
            --------------------------------------------------------------
case of any capital reorganization of the capital stock of the Company (other
than a combination, reclassification, exchange or subdivision of shares
otherwise provided for herein), or any merger or consolidation of the Company
with or into another corporation, or the sale of all or substantially all the
assets of the Company then, and in each such case, as a part of such
reorganization, merger, consolidation, sale or transfer, lawful provision shall
be made so that the Holder of this Warrant shall thereafter be entitled to
receive upon exercise of this Warrant, during the period specified herein and
upon payment of the Purchase Price then in effect, the number of shares of stock
or other securities or property of the successor corporation resulting from such
reorganization, merger, consolidation, sale or transfer that a holder of the
shares deliverable upon exercise of this Warrant would have been entitled to
receive in such reorganization, consolidation, merger, sale or transfer if this
Warrant had been exercised immediately before such reorganization, merger,
consolidation, sale or transfer, all subject to further adjustment as provided
in this Section 4. The foregoing provisions of this Section 4.4 shall similarly
        ---------                                   -----------
apply to successive reorganizations, consolidations, mergers, sales and
transfers and to the stock or securities of any other corporation that are at
the time receivable upon the exercise of this Warrant. If the per-share
consideration payable to the Holder hereof for shares in connection with any
such transaction is in a form other than cash or marketable securities, then the
value of such consideration shall be determined in good faith by the Company's
Board of Directors. In all events, appropriate adjustment (as determined in good
faith by the Company's Board of Directors) shall be made in the application of
the provisions of this Warrant with respect to the rights and interests of the
Holder after the transaction, to the end that the provisions of this Warrant
shall be applicable after that event, as near as reasonably may be, in relation
to any shares or other property deliverable after that event upon exercise of
this Warrant.

                                       4
<PAGE>

        4.5 Conversion of Common Stock. In case all or any portion of the
            --------------------------
authorized and outstanding shares of Common Stock of the Company are redeemed or
converted or reclassified into other securities or property pursuant to the
Company's Certificate of Incorporation or otherwise, or the Common Stock
otherwise ceases to exist, then, in such case, the Holder of this Warrant, upon
exercise hereof at any time after the date on which the Common Stock is so
redeemed or converted, reclassified or ceases to exist (the "Termination Date"),
                                                             ----------------
shall receive, in lieu of the number of shares of Common Stock that would have
been issuable upon such exercise immediately prior to the Termination Date, the
securities or property that would have been received if this Warrant had been
exercised in full and the Common Stock received thereupon had been
simultaneously converted immediately prior to the Termination Date, all subject
to further adjustment as provided in this Warrant. Additionally, the Purchase
Price shall be immediately adjusted to equal the quotient obtained by dividing
(x) the aggregate Purchase Price of the maximum number of shares of Common Stock
for which this Warrant was exercisable immediately prior to the Termination Date
by (y) the number of shares of Common Stock of the Company for which this
Warrant is exercisable immediately after the Termination Date, all subject to
further adjustment as provided herein.

        4.6 Issuances Below Market. In case the Company shall issue or sell (a)
            ----------------------
Common Stock, (b) rights, warrants or options entitling the holders thereof to
subscribe for or purchase shares of Common Stock or (c) any security convertible
into Common Stock, in each case at a price, or having an exercise or conversion
price, per share less than the Purchase Price (excluding any issuance for which
an appropriate and full adjustment has been made pursuant to Section 4.2), the
Purchase Price shall be immediately reduced by multiplying the Purchase Price by
a fraction of which (A) the numerator shall be the number of shares of Common
Stock outstanding immediately prior to such issuance or sale plus the number of
shares of Common Stock which the aggregate consideration received or receivable
(I) for the total number of shares of Common Stock, rights, warrants or options
or convertible securities so issued or sold and (II) upon the exercise or
conversion of all such rights, warrants, options or securities, would purchase
at such Purchase Price, and (B) the denominator shall be the number of shares of
Common Stock outstanding immediately prior to such issuance plus (without
duplication) the number of shares of Common Stock subject to all such rights,
warrants, options and convertible securities. The issuance of any shares of
Common Stock or other rights, warrants, options or convertible securities
pursuant to (a) the effectuation of a split or subdivision of the outstanding
shares of Common Stock or a dividend or other distribution payable in additional
shares of Common Stock or other securities or rights convertible into, or
entitling the holder thereof to receive directly or indirectly, additional
shares of Common Stock, (b) Common Stock (or securities exercisable or
convertible into Common Stock) issuable or issued to employees, consultants or
directors of the Company after the Purchase Date directly or pursuant to a stock
option plan or restricted stock plan approved by the Board of Directors of the
Company, (c) securities issued pursuant to the conversion or exercise of
convertible or exercisable securities outstanding or deemed outstanding on the
date of this Warrant, and (d) securities issued or issuable in connection with
the acquisition, merger, consolidation, or other business combination by or of
the Company with, by, or of any person, shall not be deemed to constitute an
issuance or sale to which this Section 4.6 applies.

     5. CERTIFICATE AS TO ADJUSTMENTS. In each case of any adjustment in the
Purchase Price, or number or type of shares issuable upon exercise of this
Warrant, the Chief Financial Officer or Controller of the Company shall compute
such adjustment in accordance with the terms of this Warrant and prepare a
certificate setting forth such adjustment and showing in detail the facts upon
which such adjustment is based, including a statement of the adjusted Purchase

                                       5
<PAGE>

Price. The Company shall promptly send (by facsimile and by either first class
mail, postage prepaid or overnight delivery) a copy of each such certificate to
the Holder.

     6. LOSS OR MUTILATION. Upon receipt of evidence reasonably satisfactory to
the Company of the ownership of and the loss, theft, destruction or mutilation
of this Warrant, and of indemnity reasonably satisfactory to it, and (in the
case of mutilation) upon surrender and cancellation of this Warrant, the Company
will execute and deliver in lieu thereof a new Warrant of like tenor as the
lost, stolen, destroyed or mutilated Warrant.

     7. RESERVATION OF COMMON STOCK. The Company hereby covenants that at all
times there shall be reserved for issuance and delivery upon exercise of this
Warrant such number of shares of Common Stock or other shares of capital stock
of the Company as are from time to time issuable upon exercise of this Warrant
and, from time to time, will take all steps necessary to amend its Certificate
of Incorporation to provide sufficient reserves of shares of Common Stock
issuable upon exercise of this Warrant, including, without limitation, engaging
in best efforts to obtain the requisite stockholder approval of any necessary
amendment to the Certificate of Incorporation. All such shares shall be duly
authorized, and when issued upon such exercise, shall be validly issued, fully
paid and non-assessable, free and clear of all liens, security interests,
charges and other encumbrances or restrictions on sale and free and clear of all
preemptive rights, except encumbrances or restrictions arising under federal or
state securities laws. Issuance of this Warrant shall constitute full authority
to the Company's Officers who are charged with the duty of executing stock
certificates to execute and issue the necessary certificates for shares of
Common Stock upon the exercise of this Warrant.

     8. TRANSFER AND EXCHANGE. Subject to the terms and conditions of this
Warrant and compliance with all applicable securities laws, this Warrant and all
rights hereunder may be transferred to any Registered Holder's parent,
subsidiary or affiliate, or, if the Registered Holder is a partnership, to any
partner of such Registered Holder, or, if the Registered Holder is a limited
liability company, to any member of such Registered Holder, in whole or in part,
on the books of the Company maintained for such purpose at the principal office
of the Company referred to above, by the Registered Holder hereof in person, or
by duly authorized attorney, upon surrender of this Warrant properly endorsed
and upon payment of any necessary transfer tax or other governmental charge
imposed upon such transfer. Upon any permitted partial transfer, the Company
will issue and deliver to the Registered Holder a new Warrant or Warrants with
respect to the shares of Common Stock not so transferred. Each taker and holder
of this Warrant, by taking or holding the same, consents and agrees that when
this Warrant shall have been so endorsed, the person in possession of this
Warrant may be treated by the Company, and all other persons dealing with this
Warrant, as the absolute owner hereof for any purpose and as the person entitled
to exercise the rights represented hereby, any notice to the contrary
notwithstanding; provided, however that until a transfer of this Warrant is duly
registered on the books of the Company, the Company may treat the Registered
Holder hereof as the owner for all purposes.

     9. RESTRICTIONS ON TRANSFER. The Holder, by acceptance hereof, agrees that,
absent an effective registration statement filed with the Securities and
Exchange Commission (the "SEC") under the Securities Act of 1933, as amended
                          ---
(the "Securities Act") covering the disposition or sale of this Warrant or the
      --------------
Common Stock issued or issuable upon exercise hereof, as the case may be, and
registration or qualification under applicable state securities laws, such
Holder will not sell, transfer, pledge, or hypothecate any or all of this
Warrant or such Common Stock, as the case may be, unless either (i) the Company
has received an opinion of counsel, in form and substance reasonably
satisfactory to the Company, to the effect that such registration is not
required

                                       6
<PAGE>

in connection with such disposition, (ii) the sale of such securities is made
pursuant to SEC Rule 144or (iii) such sale or transfer is to the Holder's
parent, subsidiary or affiliate, or, if the Holder is a partnership, to any
partner of such Holder, or, if the Holder is a limited liability company, to any
member of such Holder, pursuant to an exemption under the Securities Act.

     10. COMPLIANCE WITH SECURITIES LAWS. By acceptance of this Warrant, the
Holder hereby represents, warrants and covenants that any shares of stock
purchased upon exercise of this Warrant shall be acquired for investment only
and not with a view to, or for sale in connection with, any distribution
thereof; that the Holder has had such opportunity as such Holder has deemed
adequate to obtain from representatives of the Company such information as is
necessary to permit the Holder to evaluate the merits and risks of its
investment in the Company; that the Holder is able to bear the economic risk of
holding such shares as may be acquired pursuant to the exercise of this Warrant
for an indefinite period; that the Holder understands that the shares of stock
acquired pursuant to the exercise of this Warrant will not be registered under
the Securities Act (unless otherwise required pursuant to exercise by the Holder
of the registration rights, if any, granted to the Registered Holder) and will
be "restricted securities" within the meaning of Rule 144 under the Securities
Act and that the exemption from registration under Rule 144 will not be
available for at least one (1) year from the date of exercise of this Warrant,
subject to any special treatment by the SEC for exercise of this Warrant
pursuant to Section 2.2, and even then will not be available unless a public
            -----------
market then exists for the stock, adequate information concerning the Company is
then available to the public, and other terms and conditions of Rule 144 are
complied with; and that all stock certificates representing shares of stock
issued to the Holder upon exercise of this Warrant or upon conversion of such
shares may have affixed thereto a legend substantially in the following form:

         THE SECURITIES  REPRESENTED  HEREBY HAVE NOT BEEN REGISTERED  UNDER THE
         SECURITIES ACT OF 1933, AS AMENDED (THE "SECURITIES ACT"), OR UNDER THE
         SECURITIES  LAWS  OF  ANY  STATE.   THESE  SECURITIES  ARE  SUBJECT  TO
         RESTRICTIONS ON  TRANSFERABILITY  AND RESALE AND MAY NOT BE TRANSFERRED
         OR RESOLD EXCEPT AS PERMITTED  UNDER THE ACT AND THE  APPLICABLE  STATE
         SECURITIES  LAWS,  PURSUANT TO  REGISTRATION  OR  EXEMPTION  THEREFROM.
         INVESTORS  SHOULD  BE  AWARE  THAT  THEY  MAY BE  REQUIRED  TO BEAR THE
         FINANCIAL  RISKS OF THIS  INVESTMENT FOR AN INDEFINITE  PERIOD OF TIME.
         THE  ISSUER OF THESE  SECURITIES  MAY  REQUIRE AN OPINION OF COUNSEL IN
         FORM AND  SUBSTANCE  SATISFACTORY  TO THE ISSUER TO THE EFFECT THAT ANY
         PROPOSED  TRANSFER  OR  RESALE  IS IN  COMPLIANCE  WITH THE ACT AND ANY
         APPLICABLE STATE SECURITIES LAWS.

     11. NO RIGHTS OR LIABILITIES AS STOCKHOLDERS. This Warrant shall not
entitle the Holder to any voting rights or other rights as a stockholder of the
Company. In the absence of affirmative action by such Holder to purchase Common
Stock by exercise of this Warrant or Common Stock upon conversion thereof, no
provisions of this Warrant, and no enumeration herein of the rights or
privileges of the Holder hereof shall cause such Holder hereof to be a
stockholder of the Company for any purpose.

     12. REPRESENTATIONS AND WARRANTIES OF THE COMPANY. The Company hereby
represents and warrants to Holder that:

                                       7
<PAGE>

        12.1 Due Authorization; Consents. All corporate action on the part of
             ---------------------------
the Company, its officers, directors and shareholders necessary for (a) the
authorization, execution and delivery of, and the performance of all obligations
of the Company under, this Warrant, and (b) the authorization, issuance,
reservation for issuance and delivery of all of the Common Stock issuable upon
exercise of this Warrant, has been duly taken. This Warrant constitutes a valid
and binding obligation of the Company enforceable in accordance with its terms,
subject, as to enforcement of remedies, to applicable bankruptcy, insolvency,
moratorium, reorganization and similar laws affecting creditors' rights
generally and to general equitable principles. All consents, approvals and
authorizations of, and registrations, qualifications and filings with, any
federal or state governmental agency, authority or body, or any third party,
required in connection with the execution, delivery and performance of this
Warrant and the consummation of the transactions contemplated hereby and
thereby, including, without limitation, the issuance and delivery of all of the
Common Stock issuable upon exercise of this Warrant, have been obtained.

        12.2 Organization. The Company is a corporation duly organized, validly
             ------------
existing and in good standing under the laws of the State of Delaware and has
all requisite corporate power to own, lease and operate its property and to
carry on its business as now being conducted and as currently proposed to be
conducted.

        12.3 Valid Issuance of Stock. The outstanding shares of the capital
             -----------------------
stock of the Company are duly and validly issued, fully paid and non-assessable,
and such shares, and all outstanding options and other securities of the
Company, have been issued in full compliance with the registration and
prospectus delivery requirements of the Securities Act and the registration and
qualification requirements of all applicable state securities laws, or in
compliance with applicable exemptions therefrom, and all other provisions of
applicable federal and state securities laws, including without limitation,
anti-fraud provisions.

        12.4 Governmental Consents. All consents, approvals, orders,
             ---------------------
authorizations or registrations, qualifications, declarations or filings with
any federal or state governmental authority on the part of the Company required
in connection with the consummation of the transactions contemplated herein,
including, without limitation, those under the Securities Act and all applicable
state securities laws, shall have been obtained prior to and be effective as of
the Effective Date.

        12.5 Listing on Nasdaq or Securities Exchange. The Company shall use its
             ----------------------------------------
best efforts to list any shares of Common Stock issuable upon exercise of this
Warrant on Nasdaq or such other national securities exchange on which shares of
Common Stock are then listed. The Company will at its expense cause all shares
of Common Stock issued upon exercise of this Warrant to be listed on Nasdaq
and/or such other national securities exchange on which shares of Common Stock
are then listed at the time of such issuance and shall maintain such listing.

        12.6 No Impairment. The Company further covenants that it will not, by
             -------------
amendment of its Certificate of Incorporation or through reorganization,
consolidation, merger, dissolution or sale of assets, or by any other voluntary
act, avoid or seek to avoid the observance or performance of any of the
covenants, stipulations or conditions to be observed or performed hereunder by
the Company.

     13. NOTICES. Except as may be otherwise provided herein, all notices,
requests, waivers and other communications made pursuant to this Agreement shall
be in writing and shall be conclusively deemed to have been duly given (a) when
hand delivered to the other party; (b) when received when sent by facsimile at
the address and number set forth below; (c) three business days

                                       8
<PAGE>

after deposit in the U.S. mail with first class or certified mail receipt
requested postage prepaid and addressed to the other party as set forth below;
or (d) the next business day after deposit with a national overnight delivery
service, postage prepaid, addressed to the parties as set forth below with
next-business-day delivery guaranteed, provided that the sending party receives
a confirmation of delivery from the delivery service provider.
<TABLE>
<S>                                            <C>

         To Holder:                            To the Company:
         GenStar Capital Partners II, LP       BioSource International, Inc.
                                               820 Flynn Road
                                               Camarillo, California 93012
                                               Telephone No.:  (805)
                                               Facsimile No.:   (805)
                                               Attention:  James Chamberlain
                                               President and Chief Executive Officer

         With copies to (which shall not       With copies to (which shall not
         constitute notice):                   constitute notice):
         Latham & Watkins                      Troop Steuber Pasich Reddick & Tobe
         505 Montgomery Street, Suite 1900     2029 Century Park East, 24th Floor y, LLP
         San Francisco, CA 94111-2562          Los Angeles, CA  90067
         Telephone No.:  (415) 391-0600        Telephone No.:  (310) 728-3222
         Facsimile No.:  (415) 395-8095        Facsimile No.:  (310) 728-2222
         Attention:  Scott Haber, Esq.         Attention: Scott W. Alderton, Esq.

</TABLE>

Each person making a communication hereunder by facsimile shall promptly confirm
by telephone to the person to whom such communication was addressed each
communication made by it by facsimile pursuant hereto but the absence of such
confirmation shall not affect the validity of any such communication. A party
may change or supplement the addresses given above, or designate additional
addresses, for purposes of this Section 14 by giving the other party written
                                ----------
notice of the new address in the manner set forth above.

     14. HEADINGS. The headings in this Warrant are for purposes of convenience
in reference only, and shall not be deemed to constitute a part hereof.

     15. LAW GOVERNING. This Warrant shall be construed and enforced in
accordance with, and governed by, the laws of the State of California, with
regard to conflict of law principles of such state.

     16. NO IMPAIRMENT. The Company will not, by amendment of its Certificate of
Incorporation or bylaws, or through reorganization, consolidation, merger,
dissolution, issue or sale of securities, sale of assets or any other voluntary
action, avoid or seek to avoid the observance or performance of any of the terms
of this Warrant, but will at all times in good faith assist in the carrying out
of all such terms and in the taking of all such action as may be necessary or
appropriate in order to protect the rights of the Registered Holder of this
Warrant against impairment. Without limiting the generality of the foregoing,
the Company (a) will not increase the par value of any shares of stock issuable
upon the exercise of this Warrant above the amount payable therefor upon such
exercise, and (b) will take all such action as may be necessary or appropriate
in order that the

                                       9
<PAGE>

Company may validly and legally issue fully paid and non-assessable shares of
Common Stock upon exercise of this Warrant.

     17. NOTICES OF RECORD DATE. In case:

         17.1 the Company shall take a record of the holders of its Common Stock
(or other stock or securities at the time receivable upon the exercise of this
Warrant), for the purpose of entitling them to receive any dividend or other
distribution, or any right to subscribe for or purchase any shares of stock of
any class or any other securities or to receive any other right; or

         17.2 of any consolidation or merger of the Company with or into another
corporation, any capital reorganization of the Company, any reclassification of
the capital stock of the Company, or any conveyance of all or substantially all
of the assets of the Company to another corporation in which holders of the
Company's stock are to receive stock, securities or property of another
corporation; or

         17.3 of any voluntary dissolution, liquidation or winding-up of the
Company; or

         17.4 of any redemption or conversion of all outstanding Common Stock;

then,  and in each such case, the Company will mail or cause to be mailed to the
Registered Holder of this Warrant a notice  specifying,  as the case may be, (i)
the date on which a record is to be taken for the purpose of such dividend,
distribution  or  right,  or  (ii)  the  date  on  which  such   reorganization,
reclassification,  consolidation, merger, conveyance, dissolution,  liquidation,
winding-up,  redemption or conversion is to take place,  and the time, if any is
to be fixed, as of which the holders of record of Common Stock or (such stock or
securities  as at the time are  receivable  upon the exercise of this  Warrant),
shall be entitled to exchange  their shares of Common Stock (or such other stock
or  securities),   for  securities  or  other  property  deliverable  upon  such
reorganization,    reclassification,    consolidation,    merger,    conveyance,
dissolution,  liquidation  or  winding-up.  The Company shall use all reasonable
efforts to ensure such notice shall be delivered at least thirty (30) days prior
to the date therein specified.

     18. SEVERABILITY. If any term, provision, covenant or restriction of this
Warrant is held by a court of competent jurisdiction to be invalid, void or
unenforceable, the remainder of the terms, provisions, covenants and
restrictions of this Warrant shall remain in full force and effect and shall in
no way be affected, impaired or invalidated.

     19. COUNTERPARTS. For the convenience of the parties, any number of
counterparts of this Warrant may be executed by the parties hereto and each such
executed counterpart shall be, and shall be deemed to be, an original
instrument.

     20. NO INCONSISTENT AGREEMENTS. The Company will not on or after the date
of this Warrant enter into any agreement with respect to its securities which is
inconsistent with the rights granted to the Holders of this Warrant or otherwise
conflicts with the provisions hereof. The rights granted to the Holders
hereunder do not in any way conflict with and are not inconsistent with the
rights granted to holders of the Company's securities under any other
agreements, except rights that have been waived.

     21. SATURDAYS, SUNDAYS AND HOLIDAYS. If the Expiration Date falls on a
Saturday, Sunday or legal holiday, the Expiration Date shall automatically be
extended until 5:00 p.m. the next business day.

                                       10
<PAGE>

     22. ENTIRE AGREEMENT. This Warrant, the Securities Purchase Agreement dated
as of January 10, 2000 by and among the Company and the Holder, and the Investor
Rights Agreement dated as of February __, 2000 by and among the Company and the
Holder, contain the sole and entire agreement and understanding of the parties
with respect to the entire subject matter of this Warrant, and any and all prior
discussions, negotiations, commitments and understandings, whether oral or
otherwise, related to the subject matter of this Warrant are hereby merged
herein.

     IN WITNESS WHEREOF, the Company has caused this Warrant to be duly executed
as of the Effective Date.

BIOSOURCE INTERNATIONAL, INC.



---------------------------------------
By
Printed Name
Title

                                       11
<PAGE>

                                    EXHIBIT A
                                    ---------
                               NOTICE OF EXERCISE

                    (To be executed upon exercise of Warrant)


BioSource International, Inc.

The  undersigned  hereby  irrevocably  elects to exercise  the right of purchase
represented by the within Warrant  Certificate for, and to purchase  thereunder,
the securities of BioSource  International,  Inc., as provided for therein,  and
(check the applicable box):

[_]  tenders  herewith payment of the exercise price in full in the form of
     cash or a certified  or official  bank check in same-day  funds in the
     amount of $____________ for _________ such securities.

[_]  Elects the [Net Issue Exercise][Easy Sale Exercise] option pursuant to
     Section 2.2 or 2.3 of the Warrant, and accordingly requests delivery of a
     net of ______________ of such securities.

Please issue a certificate or certificates for such securities in the name of,
and pay any cash for any fractional share to (please print name, address and
social security number):


Name:
            ----------------------------------------------------------------

Address:
            ----------------------------------------------------------------

Signature:
            ----------------------------------------------------------------

Note: The above signature should  correspond  exactly with the name on the first
page of this Warrant Certificate or with the name of the assignee appearing in
the assignment form below.

If said number of shares shall not be all the shares purchasable under the
within Warrant Certificate, a new Warrant Certificate is to be issued in the
name of said undersigned for the balance remaining of the shares purchasable
thereunder rounded up to the next higher whole number of shares.
<PAGE>

                                    EXHIBIT B
                                    ---------
                                   ASSIGNMENT

          (To be executed only upon assignment of Warrant Certificate)

For value received, hereby sells, assigns and transfers unto ___________________
____________________________ the within Warrant Certificate, together with all
right, title and interest therein, and does hereby irrevocably constitute and
appoint ____________________________ attorney, to transfer said Warrant
Certificate on the books of the within-named Company with respect to the number
of Warrants set forth below, with full power of substitution in the premises:
<TABLE>
<CAPTION>
--------------------------------------- ------------------------------------- ------------------------------------
        Name(s) of Assignee(s)                        Address                            # of Warrants
--------------------------------------- ------------------------------------- ------------------------------------
<S>                                     <C>                                   <C>
--------------------------------------- ------------------------------------- ------------------------------------

--------------------------------------- ------------------------------------- ------------------------------------

--------------------------------------- ------------------------------------- ------------------------------------

--------------------------------------- ------------------------------------- ------------------------------------

--------------------------------------- ------------------------------------- ------------------------------------
</TABLE>

And if said number of Warrants shall not be all the Warrants represented by the
Warrant Certificate, a new Warrant Certificate is to be issued in the name of
said undersigned for the balance remaining of the Warrants registered by said
Warrant Certificate.

             --------------------------------------------------------------

Dated:
             --------------------------------------------------------------

Signature:
             --------------------------------------------------------------

Notice: The signature to the foregoing Assignment must correspond to the name as
written upon the face of this security in every particular, without alteration
or any change whatsoever; signature(s) must be guaranteed by an eligible
guarantor institution (banks, stock brokers, savings and loan associations and
credit unions with membership in an approved signature guarantee medallion
program) pursuant to Securities and Exchange Commission Rule 17Ad-15.