<PAGE>

[CONFIDENTIAL TREATMENT REQUESTED. CONFIDENTIAL PORTIONS OF THIS AGREEMENT HAVE
BEEN REDACTED AND HAVE BEEN SEPARATELY FILED WITH THE COMMISSION.]

                                                                   Exhibit 10.11

                    FIRST AMENDMENT TO THE AGREEMENT ENTITLED
                  "ASSIGNMENT OF PATENT RIGHTS AND TECHNOLOGY"
                       ENTERED INTO AUGUST 1, 1991 BETWEEN
         THE UNIVERSITY OF TEXAS, ADAM HELLER, AND E. HELLER AND COMPANY
                                (THE "AMENDMENT")

        WHEREAS, under the August 1, 1991, agreement entitled "Assignment of
Patent Rights and Technology" (hereafter "Assignment"), and Exhibit A hereto,
the Board of Regents of the University of Texas System on behalf of The
University of Texas at Austin (hereafter collectively referred to as "Assignor")
assigned to E. Heller and Company ("Assignee") its right, title, and interest
in, to, and under the Patent Rights and the Technology (as such terms are
defined in the Assignment); and

        WHEREAS, under Paragraph 3.1 of the Assignment the parties agreed that
in return for its assignment of rights, Assignor would be compensated as noted
in the event Patent Rights, the Technology, and the Know-How are sold, licensed,
or otherwise disposed of for value to a third party or parties; and under
Paragraph 3.2 of the Assignment the parties also agreed that Assignor would be
compensated for a to-be-negotiated amount should Assignee elect to itself
manufacture, sell, or otherwise dispose of for value Licensed Products and
Licensed Processes (as such terms are defined in the Assignment) and which are
derived from the Patent Rights, the Technology, and the Know-How; and

        WHEREAS, since the effective date of the Assignment, Assignee has
invested approximately *** in research and development of biosensors
within the scope of the Technology, Patent Rights, and Know-How, and estimates
that an additional investment of approximately *** will be necessary to
complete product development and enter into clinical trials, and Assignee, on
behalf of itself and its Affiliates (as defined below), has now elected to
manufacture, sell, or otherwise dispose of for value, including sublicensing,
the Patent Rights, Technology, Know-How, Licensed Products, and Licensed
Processes; and

        WHEREAS, Assignor and Assignee now desire to amend Paragraphs 3.1 and
3.2 and certain other paragraphs of the Assignment and specifically set out
herein the mutually agreed compensation due Assignor for Assignee's disposition
for value of the Patent Rights, the Technology, the Know-How, Licensed Products
and Licensed Processes.

        NOW, THEREFORE, in consideration of the covenants and agreements
herein contained the parties agree as follows:

1.      All capitalized terms not defined in this Amendment shall have the
        meaning given to them in the Assignment.

2.      Section 1.2 is amended to read in its entirety as follows:

        1.2  Patent Rights. The term "Patent Rights" as used herein shall mean
             -------------
        Assignor's rights in all inventions and intellectual property disclosed
        in the patent applications patents and other invention disclosures
        listed on Exhibit B and Exhibit C hereto, including all U.S. and


*** Confidential treatment requested
<PAGE>

         foreign patent applications including provisional applications, and all
         divisions, continuations, continuations in-part, and substitutions
         thereof; all foreign patent applications corresponding to the preceding
         applications; and all U.S. and foreign patents issuing on any of the
         preceding applications, including extensions, reissues, and re-
         examinations.

3.       Section 1.3 is amended to read in its entirety as follows:

         1.3   Know-How. The term "Know How" as used herein shall mean
               --------
         Assignor's rights in all ideas, inventions, data, trade secrets,
         instructions, processes, formulas, chemical, pharmacological,
         toxicological, pharmaceutical, physical and analytical, manufacturing
         (including but not limited to processes, yields, reagants and
         conditions relating to compound manufacture) data and information,
         owned or controlled by Assignor existing as of the last signature date
         of this Assignment which are generally not known, and which are
         necessary or useful for the manufacture of Licensed Products. Excluded
         from Know-How are any inventions included with Patent Rights.

4.       Section 1.4 is amended to read in its entirety as follows:

         1.4   Licensed Products. The term "Licensed Products" as used herein
               -----------------
         shall mean any product which (i) is covered by a Valid Claim in the
         country such product is made or sold, or (ii) incorporates in material
         part or is made using the Technology, Know-How, Licensed Processes or
         Other Intellectual Property. For the avoidance of doubt, subject to the
         foregoing, a Licensed Product unit shall mean any instrument or
         associated disposable.

5.       Article 1 is amended to add new Section 1.7 as follows:

         1.7   Affiliate. The term "Affiliate" as used herein shall mean any
               ---------
         corporation or other entity which is directly or indirectly
         controlling, controlled by or under the common control with a party
         hereto. For the purposes of this definition, "control" shall mean the
         direct or indirect ownership of at least fifty percent (50%) of the
         outstanding shares or other voting rights of the subject entity to
         elect directors, or if not meeting the preceding, any entity owned or
         controlled by or owning or controlling at the maximum control or
         ownership right permitted in the country where such entity exists.

6.       Article 1 is amended to add new Section 1.8 as follows:

         1.8   Net Sales. The term "Net Sales" as used herein shall mean the
               ---------
         gross revenues received by Assignee, its Affiliates or sublicensees
         from sales of Licensed Products, less (i) normal and customary rebates,
         and cash and trade discounts, actually taken, (ii) sales, value-added,
         use and/or other excise taxes or duties actually paid, (iii) outbound
         transportation charges prepaid or allowed, (iv) import and/or export
         duties actually paid, and (v) amounts allowed or credited due to
         returns.

7.       Article 1 is amended to add new Section 1.9 as follows:

         1.9   Valid Claim. The term "Valid Claim" as used herein shall mean (i)
               -----------
         a claim of an issued and unexpired patent included within the Patent
         Rights which has not been disclaimed

                                      -2-
<PAGE>

         or admitted to be invalid or unenforceable through reissue or
         otherwise, or (ii) a claim of a pending application within the Patent
         Rights provided that not more than five (5) years have elapsed from the
         date the claim takes priority for filing purposes.

8.       Article 1 is amended to add new Section 1.10 as follows:

         1.10  Dominating Patent. The term "Dominating Patent" as used herein
               -----------------
         shall mean an unexpired patent which is owned by a third party covering
         Licensed Products made and sold by Assignee or its sublicensees under
         circumstances such that Assignee or its sublicensee has no commercially
         reasonable alternative but to obtain a license under such patent in
         order to make, use or commercialize a Licensed Product.

9.       Article 1 is amended to add new Section 1.11 as follows:

         1.11  Other Intellectual Property. The term "Other Intellectual
               ---------------------------
         Property" as used herein shall mean Board's rights in any biosensor
         related inventions (whether or not patentable), improvements,
         discoveries, developments, original works of authorship, software,
         trade secrets, Know-How made, conceived, reduced to practice or
         otherwise developed, by an employee of Assignor pursuant to either (i)
         a consulting agreement entered into by such employee and EHC or its
         sublicensee; or (ii) any similar agreement between an employee of the
         University of Texas at Austin and Assignee, and all intellectual
         property rights therein and thereto, shall be subject to the Assignment
         in Section 2.1 below.

10.      Section 2.1 is amended to read in its entirety as follows:

         2.1   Assignment by Assignor. Assignor hereby transfers, assigns,
               ----------------------
         sells, and conveys to Assignee, its successors and assigns, the entire
         right, title and interest throughout the world in and to the Patent
         Rights, Technology, Know-How, and Other Intellectual Property subject
         only to: (a) any rights of the United States Government which may exist
         now or in the future due to a research funding agreement to which the
         United States Government may be a party; and (b) the terms and
         conditions of this Agreement.

11.      Article 3 is amended to read in its entirety as follows:

         3.1   Royalties. In consideration for the Assignment herein, Assignee
               ---------
         shall pay to Assignor the greater of (i) an annual minimum royalty of
         *** or (ii) royalties on Net Sales of Licensed Products sold by
         Assignee, an Affiliate or its sublicensees which are within the scope
         of a Valid Claim as follows:

                                      Numbers of Licensed
         Royalty Rate                 Products Sold Worldwide During the Term
         ------------                 ---------------------------------------

         ***  of Net Sales            1-100,000

         ***  of Net Sales            100,001-200,000

         ***  of Net Sales            200,001-and above

                                      -3-
<PAGE>

         Any minimum annual royalty payable hereunder shall be ***
         against any running royalty payable to Assignor during such year.

         3.2   Sublicense Payment. In addition to the royalties subject to
               ------------------
         Section 3.1 above, Assignee shall pay to Assignor *** received by
         Assignee and its Affiliates from licensees and sublicensees of the
         Patent Rights, Know-How, Technology or Other Intellectual Property.
         Notwithstanding the foregoing, it is understood and agreed that
         Assignor shall not be entitled to any portion of amounts received from
         licensees or sublicensees for equity in Assignee less than *** of
         fair market value, debt financing, research and development funding,
         the license or sublicense of any intellectual property other than the
         Patent Rights, Know-how, Technology or Other Intellectual Property, or
         reimbursement for patent or other expenses, and to be determined in a
         manner consistent with generally accepted accounting principles (GAAP).

         3.3   Royalties on Combination Products. In the event that a Licensed
               ---------------------------------
         Product is sold by Assignee, an Affiliate or its sublicensee in
         combination as a single product with another product whose sale and use
         are not within the scope of a Valid Claim in the country for which the
         combination product is sold, Net Sales from such sales for purposes of
         calculating the amounts due under Section 3.1 above shall be calculated
         by multiplying the Net Sales of that combination by the fraction A/(A +
         B), where A is the gross selling price of the Licensed Product sold
         separately and B is the gross selling price of the other product sold
         separately. In the event that no such separate sales are made by
         Assignee, an Affiliate or its sublicensee, Net Sales for royalty
         determination shall be as reasonably allocated by Assignee, an
         Affiliate or its sublicensee between such Licensed Product and such
         other product, based upon their relative importance and proprietary
         protection. Notwithstanding the above, in no event shall Assignor
         receive a royalty of less *** the running royalty amount per unit of
         Licensed Product due under Section 3.1 above.

         3.4   Third Party Royalty Offset. In the event Assignee, an Affiliate
               --------------------------
         or its sublicensee enters into a license agreement with any third party
         with respect to a Dominating Patent or to settle a claim of
         infringement or misappropriation of any intellectual property of a
         third party relating to the practice or use of the Technology and or
         Know-How, Assignee, an Affiliate or its sublicensee may offset any
         payments made in accordance with such license agreements against any
         amounts of running royalties owned by Assignee pursuant to Article 3
         herein up to a maximum of *** the amount otherwise due to such third
         party; provided, however that in no event will the running royalties
         due to Assignor be lower than the following:

         Royalty Rate                Number of Licensed Products Sold Worldwide
         ------------                ------------------------------------------
                                     During the Term
                                     ---------------

         ***  of Net Sales           1-100,000

         ***  of Net Sales           100,001-200,000

         ***  of Net Sales           200,001 - and above

                                      -4-

***  Confidential treatment requested
<PAGE>

         In addition, in such event, the amounts due to Assignor under Section
         3.2 above shall be reduced by an amount equal to *** the amount paid
         to such third party.

         3.5   One Royalty. No more than one royalty payment shall be due with
               -----------
         respect to a sale of a particular Licensed Product. No multiple
         royalties shall be payable because any Licensed Product, or its
         manufacture, use or sale is covered by more than one Valid Claim. No
         royalty shall be payable under this Article 3 with respect to Licensed
         Products distributed for use in research and/or development, in
         clinical trials or as promotional samples.

         3.6   Royalty Term. Royalties due under this Article 3 shall be payable
               ------------
         on a country-by-country and Licensed Product-by-Licensed Product basis
         until the expiration of the last-to-expire issued Valid Claim covering
         such Licensed Product in such country, or if no such Valid Claim issued
         in a country, until the fifth anniversary of the first commercial sale
         of a Licensed Product in such country.

         3.7   In the event Assignee engages in litigation and/or settlement
         negotiations to advance a claim it makes that a third party is
         infringing on Licensed Products and/or Licensed Processes, ***
         on any litigation or settlement recovery by Assignee will be paid to
         Assignor; provided, however, that Assignee shall be entitled to first
         deduct reasonable and documented attorney's and other professional
         fees, expert fees, court costs, and other reasonable expenses incurred
         by Assignee in such litigation and/or settlement activities.

12.      Section 4.2 is amended to read in its entirety as follows:

         4.2   Payments and Reports. The first annual minimum royalty payment of
               --------------------
         *** shall be due and payable to Assignor within 30 days of the date of
         the last signature on this Amendment. Thereafter, minimum annual
         royalty payments, when due, shall be due and payable within thirty (30)
         days after the end of the applicable year. All running royalties
         payable hereunder by Assignee shall be due within thirty (30) days
         following the end of each respective semi-annual reporting period,
         which ending dates shall be June 30 and December 31 for each successive
         year. Each report will accurately identify income and expenses in
         sufficient form and detail so as to enable Assignor to determine the
         royalties due for such semi-annual period, and shall be mailed to the
         following: Office of the Executive Vice President and Provost, The
         University of Texas at Austin, Main Building 201, Austin, Texas 78712-
         1111, ATTN: Patricia C. Ohlendorf, with a copy to the Office of
         Technology Licensing and Intellectual Property at the address listed in
         16.3 herein. Checks shall be made payable to The University of Texas at
         Austin. Except as expressly provided herein, all amounts payable
         hereunder shall be payable in United States dollars without deductions
         for taxes, assessments, fees, or charges of any kind.

13.      Article 15 is amended to read in its entirety as follows:

         15.1  Assignment. Neither party may assign this Assignment without the
               ----------
         prior consent of the other, which consent shall not be unreasonably
         withheld; provided, however, Assignee may assign this Assignment in
         connection with a transfer of all or substantially all of its

***  Confidential treatment requested

                                      -5-


<PAGE>

     assets relating to this Assignment, whether by sale, merger, operation of
     law or otherwise. This Assignment shall be binding upon and inure to the
     benefit of the parties and their successors and assigns.

14.  New Article 16 is added as follows:

     MISCELLANEOUS
     -------------

     16.1  Affiliate Rights/Obligations. Assignee shall have the right to extend
           ----------------------------
     the provisions of the Assignment regarding the right to manufacture, sell,
     or otherwise dispose of for value, including licensing and sublicensing,
     the Patent Rights, Technology, Know-How, Other Intellectual Property,
     Licensed Products, and Licensed Processes, to any Affiliate, provided such
     Affiliate consents to be bound by the Assignment to the same extent as
     Assignee. Further, Assignee shall have the right to extend the provisions
     of this Amendment in their entirety to any Affiliate, provided such
     Affiliate consents to be bound by the applicable provisions of this
     Amendment to the same extent as Assignee.

     16.2  Entire Agreement. The Assignment and this Amendment constitute the
           ----------------
     entire and only agreement between the parties with respect to the Patent
     Rights, Technology, Know-How, Other Intellectual Property, Licensed
     Products, and Licensed Processes, and all other prior negotiations,
     representations, agreements, and understandings are superseded hereby. No
     agreement altering or supplementing the terms of the Assignment may be made
     except by means of a written document signed by a duly authorized
     representative of each party.

     16.3. Notices. Any notices under this Assignment or shall be given by
           -------
     prepaid, first class, certified mail, return receipt requested, addressed
     as follows (or such other address as may be given from time to time under
     the terms of this provision):

<TABLE>
     <S>                                            <C>
     in the case of Assignor -                       with a copy to -
     BOARD OF REGENTS                                OFFICE OF THE EXECUTIVE
     The University of Texas System                  VICE PRESIDENT & PROVOST
     201 West 7th Street                             The University of Texas at Austin
     Austin, Texas  78701                            Austin, Texas 78712-1111
     ATTN: System Intellectual Property Office       ATTN:  Patricia C. Ohlendorf

                                                     with a copy to -
                                                     OFFICE OF TECHNOLOGY LICENSING AND
                                                     INTELLECTUAL PROPERTY
                                                     The University of Texas at Austin
                                                     MCC Building
                                                     Suite 1.9A
                                                     3925 West Braker Lane
                                                     Austin, Texas  78759
                                                     ATTN:  Director
</TABLE>


                                      -6-
<PAGE>

         or in the case of Assignee -

         E. HELLER AND COMPANY
         1311 Harbor Bay Parkway, Suite 1000
         Alameda, California  94502
         ATTN:  Ephraim Heller

         16.4  LIMITATION OF LIABILITY. SUBJECT TO ARTICLE XIV OF THE
               -----------------------
         ASSIGNMENT, NEITHER PARTY SHALL BE LIABLE TO THE OTHER FOR ANY SPECIAL,
         CONSEQUENTIAL, INCIDENTAL OR INDIRECT DAMAGES ARISING OUT OF THIS
         ASSIGNMENT, HOWEVER CAUSED, UNDER ANY THEORY OF LIABILITY

         16.5  Right to Independently Develop. Nothing in this Assignment will
               ------------------------------
         impair Assignee's right to independently acquire, license, develop for
         itself, or have others develop for it, technology or intellectual
         property performing the same or similar functions as the Know-How or
         the Patent Rights, or the Other Intellectual Property, or to market and
         distribute licensed products based on such other intellectual property
         and technology.

         16.6  Further Assurances. At any time or from time to time on and after
               ------------------
         the date of this Assignment, Assignor shall at the request and sole
         expense of Assignee (i) deliver to Assignee such records, data or other
         documents consistent with the provisions of this Assignment, (ii)
         execute, and deliver or cause to be delivered, all such consents,
         documents or further instruments of transfer or license, and (iii) take
         or cause to be taken all such actions, as Assignee may reasonably deem
         necessary or desirable in order for Assignee to obtain the full
         benefits of the Assignment and Amendment and the transactions
         contemplated hereby.

         16.7  Severability. In the event that any provisions of this Assignment
               ------------
         are determined to be invalid or unenforceable by a court of competent
         jurisdiction, the remainder of the Assignment shall remain in full
         force and effect without said provision. The parties shall in good
         faith negotiate a substitute clause for any provision declared invalid
         or unenforceable which shall most nearly approximate the intent of the
         parties in entering this Assignment.

         16.8  Modification; Waiver. This Assignment may not be altered, amended
               --------------------
         or modified in any way except by a writing signed by both parties. The
         failure of a party to enforce any provision of the Assignment shall not
         be construed to be a waiver of the right of such party to thereafter
         enforce that provision or any other provision or right.

15.      Except as specifically modified or amended hereby, the Assignment shall
         remain in full force and effect and, as so modified or amended, is
         hereby ratified, confirmed and approved. No provision of this Amendment
         may be modified or amended except expressly in a writing signed by both
         parties nor shall any terms be waived except expressly in a writing
         signed by the party charged therewith. This Amendment shall be governed
         in accordance with the laws of the State of Texas, without regard to
         principles of conflicts of laws.

                                      -7-
<PAGE>

         IN WITNESS WHEREOF, this Amendment effective as of March 19, 1998, is
hereby executed by the duly authorized representatives of the parties on the
date indicated below in duplicate, each of which shall be deemed an original and
together shall form one and the same instrument.

BOARD OF REGENTS OF THE
UNIVERSITY OF TEXAS SYSTEM                       E. HELLER AND COMPANY

/s/ Ray Farabee                                  /s/ Ephraim Heller
---------------------------------------          -------------------------------
Ray Farabee                                      Ephraim Heller
Vice Chancellor and General Counsel              President
Date:   4/7/98                                   Date:   3/19/98
     ----------------------------------               --------------------------

APPROVED AS TO CONTENT                           ADAM HELLER

/s/ Patricia C. Ohlendorf                        /s/ Adam Heller
---------------------------------------          -------------------------------
Patricia C. Ohlendorf                            Adam Heller
Counsel to the President                         Date:   3/19/98
                                                      --------------------------
Vice Provost
Date:   3/27/98
     ----------------------------------

APPROVED AS TO FORM

/s/ Georgia Harper
---------------------------------------

Name: Georgia Harper
     ----------------------------------
Office of General Counsel
Date: 4/7/98
     ----------------------------------

                                      -8-
<PAGE>

                                   EXHIBIT B

                                 (See attached)
<PAGE>

COLLEGE OF ENGINEERING

THE UNIVERSITY OF TEXAS AT AUSTIN

Department of Chemical Engineering, Austin, Texas 78712-1062


Adam Heller                                                     January 6, 1997
Ernest Cockrell, Sr. Chair
Telephone:      (512) 471-8874
FAX:            (512) 471-8799
e-mail:         heller@che.utexas.edu

To:             Dr. Paulette Braeutigam, Technology
                    Licensing Coordinator

From:           Adam Heller


Dear Paulette:

This letter follows your discussion with Ephraim wherein you agreed to cover in
the current agreement between The University of Texas at Austin and E. Heller &
Company all presently existing intellectual property relating to biosensors that
originated in my group. These include the following:

US Patents Issued:
-----------------

The already issued patents are:

US 5,543,326 - Biosensor including chemically modified enzymes.

US 5,356,786 - Interferant eliminating biosensor

US 5,320,725 - Electrode and Method for the detection of hydrogen peroxide.

US 5,264,105 - Enzyme electrodes.

US 5,264,104 - Enzyme electrodes.

US 5,262,305 - Interferant eliminating biosensor.

US 5,262,035 - Enzyme electrodes.

<PAGE>

US Patents Currently Prosecuted:
-------------------------------

(1)    ***

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<PAGE>

***

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                                      -3-
<PAGE>

***

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                                      -4-
<PAGE>

***

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                                      -5-
<PAGE>


***

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                                      -6-


<PAGE>

***


With my regards and best wishes for 1997.



Adam Heller

AH:n

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                                      -7-