CO-SALE AGREEMENT


        THIS CO-SALE AGREEMENT (the "Co-Sale Agreement") is entered into as of
November __, 2000, by and among Toxco., a _______ corporation (the "Company"),
CombiMatrix Corporation, a Delaware corporation ("CombiMatrix") and Acacia
Research Corporation, a Delaware corporation ("Acacia"; together with
CombiMatrix, the "Investors" each of which is herein referred to as an
"Investor".)

                                    RECITALS

        WHEREAS, the Company and the Investors are acquiring the Common Stock of
the Company (the "Common Stock");

        WHEREAS, the rights and obligations granted in this Co-Sale Agreement
are a condition to the effectiveness of the acquisition of such Common Stock;

        NOW, THEREFORE, the parties agree as follows:

5. Definitions. For purposes of this Co-Sale Agreement, the following terms
shall have the following meanings.

        5.1    "Act" means the Securities Act of 1933, as amended.

        5.2    "Common Stock" means the common stock, par value $.001 per share,
               of the Company.

        5.3    "Common Stock Equivalents" means and includes all shares of
               Common Stock issued and outstanding at the relevant time plus (i)
               all shares of Common Stock that may be issued upon exercise of
               any options, warrants and other rights of any kind that are then
               exercisable, and (ii) all shares of Common Stock that may be
               issued upon conversion or exchange of any convertible securities
               which are by their terms then convertible into or exchangeable
               for Common Stock, including without limitation, preferred stock.

6. Investors' Right of First Offer. Before any Common Stock owned by any
Investor may be Transferred (as defined hereinafter), the other Investor shall
have a right of first offer with respect to such Common Stock, all on the terms
and conditions set forth in this Section 2.

               Each time an Investor proposes to sell, transfer or otherwise
dispose of Common Stock other than to any wholly owned subsidiary or parent of,
or to any corporation that is, within the meaning of the Act, controlling,
controlled by or under common control with, any such investor, in the case of a
limited liability company, to its members, or gift transfers to its family
members or one or more trusts for the benefit of the foregoing (in each case a
"Transfer"),

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that Investor (the "Transferring Investor") shall first make an offering of
Common Stock to the other Investor (the "Other Investor") in accordance with the
following provisions:

               (a) The Transferring Investor shall provide notice (an "Investor
Transfer Notice") to the Other Investor stating (i) its bona fide intention to
Transfer such Common Stock, (ii) the number of such Common Stock shares proposed
to be Transferred (the "Offered Stock"), (iii) the name of each proposed
transferee, and (iv) the cash price or other consideration (including the cash
equivalent value of any non-cash consideration) per share for which the
Transferring Investor proposes to transfer the Offered Stock (the "Offered
Price").

               (b) By written notification received by the Other Investor within
ten (10) calendar days of the mailing of the Investor Transfer Notice, the Other
Investor may elect to purchase or obtain, at the price and on the terms
specified in the Investor Transfer Notice, all of the Offered Stock.

               (c) If all Offered Stock which the Other Investor is entitled to
obtain pursuant to subsection 2(b) is not elected to be obtained, the
Transferring Investor may, during the sixty (60) day period following the
expiration of the period provided in subsection 2(b) hereof, offer the Offered
Stock to any person or persons, subject to Section 3 hereof, at a price not less
than, and upon terms no more favorable to the offeree than those specified in
the Investor Transfer Notice. If the Transferring Investor does not enter into
an agreement for the sale of the Common Stock within such period, or if such
agreement is not consummated within ninety (90) days of the execution thereof,
the right provided hereunder shall be deemed to be revived and such Offered
Stock shall not be transferred unless first reoffered to the Other Investor in
accordance herewith.

               (d) The right of first offer set forth in this Section 2 shall
terminate upon the completion of an underwritten public offering of shares of
Common Stock of the Company (a "Qualified Public Offering").

               (e) Any attempt by an Investor to transfer Common Stock in
violation of Section 2 hereof shall be void and the Company agrees it will not
effect such a transfer nor will it treat any alleged transferee as the holder of
such shares without the written consent of a majority in interest of the
Investors.

7.      Rights of Co-Sale

        7.1    Investors' Rights. If the Other Investor does not fully exercise
               its right of first offer pursuant to Section 2(b) hereof, such
               Other Investor shall have a right of co-sale with respect to the
               sale of any Common Stock owned by the Transferring Investor, all
               on the terms and conditions set forth in this Section 3.

        7.2    Notice of Proposed Transfer. Before a Transferring Investor may
               transfer its shares of Common Stock and after the Other Investor
               has elected not to fully exercise its right of first offer
               pursuant to Section 2, such Transferring Investor shall provide
               notice (a "Transferring Investor Notice") to the Other Investor
               stating (i) its bona



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               fide intention to Transfer such Common Stock, (ii) the number of
               shares of Offered Stock, (iii) the name of each proposed
               transferee, and (iv) the Offered Price.

        7.3    Right of Co-Sale.

                      (a)    Exercise of Right of Co-Sale. The Other Investor
                             shall have the right to participate in the
                             Transferring Investor's transfer of the Offered
                             Stock to the proposed transferee pursuant to the
                             specified terms and conditions of the Transfer as
                             set forth in the Transferring Investor Notice and
                             in accordance with the terms and conditions of the
                             Transfer as set forth in this Section 3.3 (the
                             "Right of Co-Sale"). For purposes of the preceding
                             sentence, the participation of the Other Investor
                             shall be on the same terms as the Transferring
                             Investor. To the extent the Other Investor
                             exercises his, her or its Right of Co-Sale, the
                             number of shares of Offered Stock that the
                             Transferring Investor may transfer pursuant to the
                             Transferring Investor Notice shall be
                             correspondingly reduced. The Other Investor shall
                             be responsible for its pro rata share of the
                             reasonable fees and expenses of the Transferring
                             Investor relating to the negotiation of the
                             transaction. The Right of Co-Sale of the Other
                             Investor shall be subject to the following terms
                             and conditions:

                             (i)    The Other Investor may transfer all or any
                                    part of such Investor's Common Stock that is
                                    not in excess of the number obtained by
                                    multiplying the aggregate number of shares
                                    of Common Stock constituting the Offered
                                    Stock by a fraction (A) the numerator of
                                    which is the number of shares of such
                                    Investor's Common Stock, and (B) the
                                    denominator of which is the total number of
                                    Common Stock Equivalents then owned by the
                                    Investors and investors, if any, who are
                                    parties to any other co-sale agreement with
                                    the Company and the Investors which is in
                                    existence at the time of the Transfer.

                             (ii)   By written notice received by the Company
                                    within fifteen (15) calendar days of the
                                    mailing of the Transferring Investor Notice
                                    by the Transferring Investor to the Company,
                                    the Other Investor may effect its election
                                    to participate in the transfer subject to
                                    this Section 3.3. Such written notice shall



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                                    contain the Other Investor's election to
                                    participate in the Transfer of the Offered
                                    Stock setting forth the number and type of
                                    shares that the Other Investor elects to
                                    include in the Transfer, accompanied by one
                                    or more certificates or other documentation,
                                    properly endorsed for transfer, representing
                                    those shares (if the Other Investor so
                                    elects, then such Other Investor shall be
                                    referred to in this Co-Sale Agreement as a
                                    "Participant"). The Company shall keep the
                                    Transferring Investor promptly informed at
                                    all times as to the Other Investor's
                                    election to participate.

                      (b)    Delivery of Stock Certificates and Proceeds. Upon
                             the closing of the Transfer subject to this Section
                             3.3, the stock certificate(s) or other
                             documentation representing the Common Stock to be
                             transferred shall be transferred and delivered to
                             the investor or transferee pursuant to the terms
                             and conditions specified in the Transferring
                             Investor Notice, and there shall be promptly
                             thereafter remitted to the Participant that portion
                             of the proceeds from the Transfer to which it is
                             entitled by reason of participating in the
                             Transfer.

        7.4    Offering Investor's Right to Transfer. The Transferring Investor
               may transfer that portion of the Offered Stock permitted to be
               transferred by the Transferring Investor, after application of
               the Right of Co-Sale contained in Section 3.3 hereof, to any
               person named as an investor or other transferee in the
               Transferring Investor Notice, at the Offered Price, provided that
               the transfer (i) is consummated within ninety (90) days after the
               date of the Transferring Investor Notice and (ii) is in
               accordance with all the terms of this Co-Sale Agreement. If the
               Offered Stock is transferred in accordance with the terms and
               conditions of this Co-Sale Agreement, then the transferee(s) of
               the Offered Stock shall thereafter hold the Offered Stock free of
               this Co-Sale Agreement and the Right of Co-Sale set forth herein.
               If the Offered Stock is not so transferred during such ninety
               (90) day period, then the Transferring Investor shall not
               transfer any of the Offered Stock without complying again in full
               with the provisions of this Co-Sale Agreement.

        7.5    Effect of Prohibited Transfer. In the event an Investor should
               sell any Common Stock in contravention of the co-sale rights of
               the other Investor under this Co-Sale Agreement (such an investor
               a "Violating Investor"; such a transaction a "Prohibited
               Transfer"), the other Investor, in addition to all other remedies
               available at law, in equity or hereunder, shall have the put
               option provided below, and the Violating Investor shall be bound
               by the applicable provisions of that option. Any attempt by a
               Violating Investor to transfer Common Stock in



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               violation of Section 3 hereof shall be void and the Company
               agrees it will not effect such a transfer nor will it treat any
               alleged transferee as the holder of such shares without the
               written consent of a majority in interest of the Investors.

        7.6    Put Option. Notwithstanding Section 3.5 hereof, in the event of a
               Prohibited Transfer, the other Investor shall have the right to
               sell to the Violating Investor the type and number of shares of
               Common Stock equal to the number of shares such Investor would
               have been entitled to sell to the Violating Investor or
               transferee under Section 3.3 hereof had the Prohibited Transfer
               been effected pursuant to and in compliance with the terms
               hereof. This sale shall be made on the following terms and
               conditions:

                      (a)    The price per share at which the Common Stock is to
                             be sold to the Violating Investor shall be equal to
                             the price per share (on an as-converted basis) paid
                             by the transferee to the Violating Investor in the
                             Prohibited Transfer. The Violating Investor shall
                             also reimburse the other Investor for any and all
                             fees and expenses, including legal fees and
                             expenses, incurred pursuant to the exercise or the
                             attempted exercise of the Investor's rights under
                             Section 3.

                      (b)    Within ninety (90) days after the later of the
                             dates on which the other Investor (A) receives
                             notice of the Prohibited Transfer or (B) otherwise
                             becomes aware of the Prohibited Transfer, the other
                             Investor, if exercising the option created hereby,
                             shall deliver to the Violating Investor the
                             certificate or certificates representing shares to
                             be sold, each certificate to be properly endorsed
                             for transfer.

                      (c)    The Violating Investor shall, upon receipt of the
                             certificate or certificates for the shares to be
                             sold by the Investor pursuant to this Section 3.6,
                             pay the aggregate purchase price therefor and the
                             amount of reimbursable fees and expenses, as
                             specified in Section 3.6(a), in cash or by other
                             means acceptable to the other Investor.

8.  Exceptions, Termination.

        8.1    Exceptions. Notwithstanding anything in this Co-Sale Agreement to
               the contrary, the Right of Co-Sale set forth in this Co-Sale
               Agreement shall not apply to any Transfer or series of Transfers
               of Common Stock by an Investor (i) pursuant to a reorganization
               or merger of the Company with or into any other corporation or
               entity, or a sale of all or substantially all of the assets of
               the Company, in which the Investors in the aggregate own
               immediately after such transaction less than



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               50% of the voting equity securities of the surviving entity, (ii)
               pursuant to the winding up and dissolution of the Company or
               (iii) in connection with a Qualified Public Offering.

        8.2    Termination of Rights. The Right of Co-Sale set forth herein
               shall terminate upon the earlier to occur of (i) the closing of a
               Qualified Public Offering, or (ii) the date on which this Co-Sale
               Agreement is terminated by a written agreement to such effect
               executed by the Company and holders of at least seventy five
               percent (75%) of the shares of Common Stock then held by the
               Investors.

9.      Restrictive Legend and Stop-Transfer Orders.

        9.1    Legend. The Investors understand and agree that the Company shall
               cause the legend set forth below, or a legend substantially
               equivalent thereto, to be placed upon any certificate(s)
               evidencing ownership of the Common Stock:

        THE SHARES REPRESENTED BY THIS CERTIFICATE ARE SUBJECT TO CERTAIN
        RESTRICTIONS ON TRANSFER SET FORTH IN A CO-SALE AGREEMENT ENTERED INTO
        BY THE ORIGINAL HOLDER OF THESE SHARES, THE COMPANY AND OTHER PERSONS, A
        COPY OF WHICH IS ON FILE AT THE PRINCIPAL OFFICE OF THE COMPANY. SUCH
        RESTRICTIONS ARE BINDING ON TRANSFEREES OF THESE SHARES UNDER SOME
        CIRCUMSTANCES.

        9.2    StopTransfer Instructions. The Investors agree, to ensure
               compliance with the restrictions referred to herein, that the
               Company may issue appropriate "stop transfer" certificates or
               instructions with respect to the Common Stock and that, the
               Company may make appropriate notations to the same effect in its
               records.

10.     Miscellaneous Provisions.

        10.1   Notices. All notices, requests, demands and other communications
               which are required to be or may be given under this Co-Sale
               Agreement to any party by any of the other parties shall be in
               writing and shall be deemed to have been duly given when (a)
               delivered in person, (b) the day following dispatch by a
               nationally recognized overnight courier service (such as Federal
               Express or UPS, etc.) for next day delivery, (c) five (5) days
               after dispatch by certified or registered first class mail,
               postage prepaid, return receipt requested, to the party to whom
               the same is so given or made or (d) in the case of notice sent by
               telecopy, on the date of dispatch, provided that the report
               generated by the sender's telecopy machine shows that all pages
               of such notice or other communication were properly transmitted
               to each recipient's telecopy number. Any notice or other
               communication given hereunder shall be addressed to the Company,
               at its principal offices as set forth below or to the Investors
               at their addresses indicated on the signature page hereto.



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        10.2   Binding on Successors and Assigns. This Co-Sale Agreement, and
               the rights and obligations of the parties hereunder, shall inure
               to the benefit of, and be binding upon, their respective
               successors, assigns and legal representatives.

        10.3   Severability. If one or more of the provisions of this Co-Sale
               Agreement should, for any reason, be held to be invalid, illegal
               or unenforceable in any respect, such invalidity, illegality or
               unenforceability shall not affect any other provisions of this
               Co-Sale Agreement, and such invalid, illegal or unenforceable
               provision shall be enforced to the extent permissible.

        10.4   Amendment. Any amendment, modification or waiver of this Co-Sale
               Agreement shall be effective only with the written consent of (i)
               Investors holding more than seventy five percent (75%) of the
               then outstanding Common Stock, and (ii) the Company; provided,
               however, that any person may waive, reduce or release (in whole
               or in part) any of its rights hereunder without the consent of
               any other parties hereto. Any waiver by a party of its rights
               hereunder shall be effective only if evidenced by a written
               instrument executed by a duly authorized representative of such
               party.

        10.5   Governing Law. This Co-Sale Agreement shall be governed by and
               construed in accordance with the internal laws (and not the laws
               pertaining to choice of conflict of laws) of the State of
               Delaware. The parties hereby agree that any dispute which may
               arise between them arising out of or in connection with this
               Co-Sale Agreement shall be adjudicated before a court located in
               Wilmington, Delaware and they hereby submit to the exclusive
               jurisdiction of the courts of the State of Delaware located in
               Wilmington, Delaware and of the federal courts in the State of
               Delaware with respect to any action or legal proceeding commenced
               by any party, and irrevocably waive any objection they now or
               hereafter may have respecting the venue of any such action or
               proceeding brought in such a court or respecting the fact that
               such court is an inconvenient forum, relating to or arising out
               of this Co-Sale Agreement or any acts or omissions relating to
               the sale of the securities hereunder, and consent to the service
               of process in any such action or legal proceeding by means of
               registered or certified mail, return receipt requested, in care
               of the address set forth in Section 6.1 or such other address as
               each party shall furnish in writing to the other parties.

        10.6   Obligations of Company. The Company agrees to use its best
               efforts to enforce and abide by the terms of this Co-Sale
               Agreement, to inform the Investors of any breach hereof (to the
               extent the company has knowledge thereof) and to assist the
               Investors in the exercise of their rights hereunder.

        10.7   Expenses. If any action at law or in equity is necessary to
               enforce or interpret the terms of this Co-Sale Agreement, the
               prevailing party shall be entitled to reasonable attorneys' fees,
               costs and necessary disbursements in addition to any other relief
               to which such party may be entitled.



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        10.8   Counterparts. This Co-Sale Agreement may be executed in any
               number of counterparts, each of which shall be deemed an original
               and all of which together shall constitute one and the same
               instrument.

        10.9   Facsimile Signatures. Any signature page delivered by a fax
               machine or telecopy machine shall be binding to the same extent
               as an original signature page, with regard to any agreement
               subject to the terms hereof or any amendment thereto. Any party
               who delivers such a signature page agrees to later deliver an
               original counterpart to any party which requests it.

        10.10  Entire Agreement. This Co-Sale Agreement, together with the
               exhibit hereto, constitutes the entire understanding and
               agreement of the parties with respect to the subject matter
               hereof and supersedes all prior understandings and agreements
               with respect to such subject matter.



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        IN WITNESS WHEREOF, the parties hereto have executed this Co-Sale
Agreement as of the date first written above.

                                        TOXCO, INC.,
                                        a _________ corporation


                                        By:
                                           -------------------------------------


                                        Address:



                                        INVESTORS:

                                        ACACIA RESEARCH CORPORATION, a Delaware
                                        corporation


                                        By:
                                           -------------------------------------

                                        Address:



                                        COMBIMATRIX CORPORATION, a Delaware
                                        corporation


                                        By:
                                           -------------------------------------

                                        Address:

Source: OneCLE Business Contracts.