Document:

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                                                                     Exhibit 4.6

                            FOURTH AMENDMENT TO THE
                 INFORMATION AND REGISTRATION RIGHTS AGREEMENT

        This Fourth Amendment to the Information and Registration Rights
Agreement, as amended (the "Amendment"), is made as of September 1, 1998, by and
among Biosource Technologies, Inc., a California corporation ("Biosource"), and
The Dow Chemical Company ("TDCC").

                                 R E C I T A L S

        A. Biosource and TDCC have entered into a Common Stock Warrant Agreement
dated of even date herewith (the "Warrant Agreement") whereby Biosource has
agreed to grant to TDCC a warrant to purchase Common Stock of Biosource (the
"Warrant").

        B. Biosource and TDCC desire to provide for the rights of TDCC with
respect to registration of the shares of the Common Stock purchased by TDCC
pursuant to TDCC's exercise of the Warrant according to the terms of the
Information and Registration Rights Agreement dated as of October 11, 1990, as
amended by the Amendment to the Information and Registration Rights Agreement
dated as of October 11, 1990, the Second Amendment to the Information and
Registration Rights Agreement dated as of October 10, 1991 and the Third
Amendment to the Information and Registration Rights Agreement dated as of March
20, 1998 (collectively, the "Agreement").

        C. Biosource and TDCC desire to amend the Agreement to grant to TDCC
Registration rights in accordance with the Agreement with respect to the shares
of Common Stock purchased by TDCC pursuant to the exercise of the Warrants
purchased by TDCC pursuant to the Warrant Agreement.

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        THE PARTIES AGREE AS FOLLOWS:

        1. Definitions. Unless otherwise indicated, the capitalized terms used
in this Amendment without definition shall have the same meanings as set forth
in the Agreement.

        2. Grant of Registration Rights. In accordance with Section 9 of the
Agreement, TDCC shall be considered an Investor for purposes of the Agreement
with respect to shares of Common Stock purchased by TDCC pursuant to exercise of
the Warrant.

        3. Counterparts. This Amendment may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

        4. Governing Law. This Amendment shall be governed by and construed in
accordance with the laws of the state of California applicable to contracts
entered into and wholly to be performed within the state of California.

        5. Headings. The headings of the Sections of this Amendment are for
convenience and shall not by themselves determine the interpretation of this
Agreement.

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        IN WITNESS WHEREOF, the parties hereto have executed this Amendment as
of the day and year first above written.

BIOSOURCE TECHNOLOGIES, INC.             THE DOW CHEMICAL COMPANY

By: /s/ Robert L. Erwin                  By:
   ----------------------------------       ------------------------------------
   Robert L. Erwin, Chairman of the      Name:
   Board and Chief Executive Officer          ----------------------------------
                                         Title:
                                               ---------------------------------

             Signature Page for Fourth Amendment to the Information
                        and Registration Rights Agreement<PAGE>   1
                                                                      EXIBIT 4.8

                               WARRANT TO PURCHASE

                            SERIES E PREFERRED STOCK

        THESE SECURITIES HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF
        1933. THEY MAY NOT BE SOLD, OFFERED FOR SALE, PLEDGED OR HYPOTHECATED IN
        THE ABSENCE OF A REGISTRATION STATEMENT IN EFFECT WITH RESPECT TO THE
        SECURITIES UNDER SUCH ACT OR AN OPINION OF COUNSEL SATISFACTORY TO THE
        COMPANY THAT SUCH REGISTRATION IS NOT REQUIRED OR UNLESS SOLD PURSUANT
        TO RULE 144 OF SUCH ACT.

                               DATED MAY 31, 1997

                          BIOSOURCE TECHNOLOGIES, INC.

             INCORPORATED UNDER THE LAWS OF THE STATE OF CALIFORNIA
                             VOID AFTER MAY 20, 2001

THIS CERTIFIES THAT, for value received, Bruce A. Boyd ("Holder"), is entitled
to purchase, on the terms hereof, Forty-Six Thousand Eight Hundred Seventy-Five
(46,875) shares of Series E Preferred Stock of Biosource Technologies, Inc., a
California corporation (the "Company"), at a purchase price and subject to
adjustment as set forth herein.

1.   EXERCISE OF WARRANT

        The terms asked conditions upon which this Warrant may be exercised, and
the Preferred Stock covered hereby (the "Warrant Stock") may be purchased, are
as follows:

1.1  TERM AND EXERCISABILITY OF WARRANT.

        Commencing on the Warrant Issue Date and ending on May 30, 2001, this
Warrant may be exercised in whole or in part immediately in accordance with the
terms and conditions of this warrant.

1.2  EXERCISE PRICE.

        The purchase price for the shares of Warrant Stock to be issued upon
exercise of this Warrant shall be Twelve Dollars and Sixty-Two Cents ($12.62)
per share, subject to adjustment as set forth herein. Upon issuance in
accordance with the terms hereof, the Warrant Stock will be fully paid and
nonassessable.

1.3  EXERCISE OF WARRANT.

        The exercise of the purchase rights evidenced by this Warrant shall be
effected by (a) the surrender of the Warrant, together with a duly executed copy
of the form of subscription articled hereto, to the Company at its principal
offices and (b) the delivery of the purchase price

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by check or bank draft payable to the Company's order for the number of shares
for which the purchase rights hereunder are being exercised, or any other form
of consideration approved by the Company's Board of Directors.

1.4  ISSUANCE OF SHARES.

        In the event the purchase rights evidenced by this Warrant are exercised
in whole or in part, a certificate or certificates for the purchased shares
shall be issued to Holder as soon as practicable. In the event the purchase
rights evidenced by this Warrant are exercised in part, the Company will also
issue to Holder a new warrant representing the unexercised purchase rights.

2.   FRACTIONAL SHARES

        No fractional shares shall be issued in connection with any exercise of
this Warrant. In lieu of the issuance of such fractional share, the Company
shall make a cash payment equal to the then fair market value of such fractional
share as determined by the Company's Board of Directors.

3.   RESERVATION OF PREFERRED STOCK

        The Company shall at all times reserve and keep available out of its
authorized but unissued shares of Preferred Stock, solely for the purpose of
effecting the exercise of this Warrant such number of its shares of Series E
Preferred Stock as shall from time to time be sufficient to effect the exercise
of this Warrant; and if at any time the number of authorized but unissued shares
of Series E Preferred Stock shall not be sufficient to effect the exercise of
the entire Warrant, in addition to such other remedies as shall be available to
the holder of this Warrant, the Company will use its reasonable best efforts to
take such corporate action as may, in the opinion of its counsel, be necessary
to increase its authorized but unissued shares of Preferred Stock to such number
of shares as shall be sufficient for such purposes.

4.   RESERVATION OF COMMON STOCK

        The Company shall at all times reserve and keep available out of its
authorized but unissued shares of Common Stock, solely for the purpose of
effecting any Conversion Event as defined in Article III, Section 5 of the
Company's Amended and Restated Articles of Incorporation (the "Articles") such
number of shares of Common Stock as shall be issuable with respect to the
Warrant Stock upon the occurrence of a Conversion Event; and if at any time the
number of authorized but unissued shares of Common Stock shall not be sufficient
to effect all shares subject to a Conversion Event, in addition to such other
remedies as shall be available to the holder of the Common Shares, the Company
will use its reasonable best efforts to take such corporate action as may, in
the opinion of its counsel, be necessary to increase its authorized but unissued
shares of Common Stock to such number of shares as shall be sufficient for such
purposes.

5.   PRIVILEGE OF STOCK OWNERSHIP

        Prior to the exercise of this Warrant, Holder shall not be entitled, by
virtue of

                                       2.
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holding this Warrant, to any rights of a stockholder of the Company, including
(without limitation) the right to vote, receive dividends or other
distributions, exercise preemptive rights or be notified of stockholder
meetings, and such holder shall not be entitled to any notice or other
communication concerning the business or affairs of the Company, except as
required by law.

6. LIMITATION OF LIABILITY

        No provision hereof, in the absence of affirmative action by the holder
hereof to purchase the Warrant Stock, and no mere enumeration herein of the
rights or privileges of the holder hereof, shall give rise to any liability of
such holder for the purchase price or as a stockholder of the Company, whether
such liability is asserted by the Company or by creditors of the Company.

7. NO TRANSFER

        This Warrant and all rights hereunder may not be transferred in whole or
in part without the prior written consent of the Company, and any such attempted
transfer without the Company's consent shall be null and void.

8. WAIVER OF CERTAIN DIVIDEND RIGHTS

        Holder hereby expressly waives all rights to dividends preference in
excess of Ninety-six Cents ($0.96), as a Series E Preferred Stockholder of the
Company to which Holder may be entitled under Section 2 of the Articles.

        Holder agrees that the Company may require, as a condition to any
transfer of the Series E Preferred Stock, a similar waiver from the transferee,
and that the certificates evidencing shares of Series E Preferred Stock may bear
a legend to that effect.

9. WAIVER OF CERTAIN LIQUIDATION RIGHTS

        Holder hereby expressly waives all rights to any liquidation preference
in excess of Twelve Dollars ($12.00), as a Series E Preferred Stockholder of the
Company to which Holder may be entitled under Section 3 of the Articles.

        Holder agrees that the Company may require, as a condition to any
transfer of the Series E Preferred Stock, a similar waiver from the transferee,
and that the certificates evidencing shares of Series E Preferred Stock may bear
a legend to that effect.

10. WAIVER OF CONVERSION PRICE ADJUSTMENT

        Holder hereby expressly waives all rights, as a Series E Preferred
Stockholder of the Company only, to any conversion price adjustment of the
undersigned's Series E Preferred Stock as provided in Article III, Sections 5(a)
and 5(d) of the Articles which may be caused by the issuance, existence or
exercise of the certain warrant for 100,000 shares of Series E Preferred Stock
in favor of Bay City Capital, LLC. Holder acknowledges that the adjustments to
the conversion price of the Series E Preferred Stock provided in Section 5(d) of
the Articles are

                                       3.
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applicable only after the Original Issue Date (as defined in the Articles) of
the Series E Preferred Stock, which will not occur until a share of the Series E
Preferred Stock is first issued.

        Holder agrees that the Company may require, as a condition to any
transfer of the Series E Preferred Stock, a similar waiver from the transferee,
and that the certificates evidencing shares of Series E Preferred Stock may bear
a legend to that effect.

11. PAYMENT OF TAXES

        The Company shall pay all expenses in connection with, and all taxes and
other governmental charges (other than any thereof on, based on or measured by,
the net income of the holder thereof) that may be imposed in respect of, the
issue or delivery of the Warrant Stock. The Company shall not be required,
however, to pay any tax or other charge imposed in connection with any transfer
involved in the issue of any certificate for shares of the Warrant Stock in any
name other than that of Holder, and in such case, the Company shall not be
required to issue or deliver any stock certificate until such tax or other
charge has been paid or it has been established to the Company's satisfaction
that no such tax or other charge is due.

12. LOSS, THEFT, DESTRUCTION OR MUTILATION OF WARRANT

        Upon receipt by the Company of evidence reasonably satisfactory to it of
the loss, theft, destruction or mutilation of this Warrant, and in case of loss,
theft or destruction, of indemnity or security reasonably satisfactory to the
Company, and upon reimbursement to the Company of all reasonable expenses
incidental thereto, and upon surrender and cancellation of this Warrant, if
mutilated, the Company will make and deliver a new warrant of like tenor and
dated as of such cancellation, in lieu of this Warrant.

13. SATURDAYS, SUNDAYS, HOLIDAYS

        If the last or appointed day for the taking of any action or the
expiration of any right required or granted herein shall be Saturday or Sunday
or shall be a legal holiday, then such action may be taken or such right may be
exercised, except as to the purchase price, on the next succeeding day not a
legal holiday.

14. RESTRICTED SECURITIES

14.1 INVESTMENT REPRESENTATIONS.

        Holder represents that:

14.2 PURCHASE FOR OWN ACCOUNT.

        This Warrant and the Warrant Stock (collectively, the "Securities") are
being acquired for its own account, not as a nominee or agent and not with a
view to resale or distribution of any part thereof, and it has no present
intention of selling, granting any participation in, or otherwise distributing
the same. Holder further represents that it does not have any contract,
undertaking agreement, or arrangement with any person to sell, transfer or grant

                                       4.
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participation to any third person with respect to the Securities.

14.3 RESTRICTED SECURITIES.

        Holder understands that the Securities may be characterized as
"restricted securities" under the federal securities laws inasmuch as they are
being acquired from the Company in transactions not involving a public offering
and that under such laws and applicable regulations such securities may be
resold without registration under the Securities Act of 1933, as amended (the
"Act"), only in certain limited circumstances. In this connection, Holder
represents that it is familiar with SEC Rule 144, as presently in effect and
understands the resale limitations imposed thereby and by the Act.

14.4 RESTRICTIONS ON DISPOSITION.

        Without in any way limiting the representations set forth above, Holder
agrees not to make any disposition of all or any portion of the Securities
unless and until:

There is then in effect a registration statement under the Act covering such
proposed disposition and such disposition is made in accordance with such
registration statement; or

(i) Holder shall have notified the Company of the proposed disposition and shall
have furnished the Company with a detailed statement of the circumstances
surrounding the proposed disposition, and (ii) if reasonably requested, Holder
shall have furnished Company with an opinion of counsel, reasonably satisfactory
to the Company, that such disposition will not require registration of the
Securities under the Act.

14.5 LEGENDS.

        Holder understands the instruments evidencing the Securities may bear
one or all of the following legends:

"These securities have not been registered under the Securities Act of 1933, as
amended. They may not be sold, offered for sale, pledged or hypothecated in the
absence of a registration statement in effect with respect to the securities
under such act or an opinion of counsel satisfactory to the Company that such
registration is not required or unless sold pursuant to rule 144 of such act."

"The shares evidenced by this certificate are subject to the terms and
conditions of a certain information and registration rights agreement which
includes a market stand-off agreement. Copies of the agreement may be obtained
upon a written request to the Company's secretary."

"The shares evidenced by this certificate are subject to a waiver of certain
dividend rights executed by the registered holder hereof with respect to the
issuance of certain securities of the Company. A copy of the waiver of
conversion price adjustment may be obtained upon written request to the
Company's secretary."

"The shares evidenced by this certificate are subject to a waiver of certain
liquidation rights

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executed by the registered holder hereof with respect to the issuance of certain
securities of the Company. A copy of the waiver of conversion price adjustment
may be obtained upon written request to the Company's secretary."

"The shares evidenced by this certificate are subject to a waiver of conversion
price adjustment executed by the registered holder hereof with respect to the
issuance of certain securities of the Company. A copy of the waiver of
conversion price adjustment may be obtained upon written request to the
Company's secretary."

Any legend required by applicable state law.

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15.  GOVERNING LAW

        This Warrant shall be governed by and construed and enforced in
accordance with the laws of the State of California.

                                            BIOSOURCE TECHNOLOGIES, INC.

                                            By
                                                   Robert L. Erwin
                                                   Chairman of the Board

Dated:  ________________ , 1997

                                       7.

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