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                                                                    EXHIBIT 10.5



                           CHANGE OF CONTROL AGREEMENT

        This Change of Control Agreement (the "Agreement") is made and entered
into effective as of January 19, 2000, by and between Joseph M. Limber (the
"Employee") and ACLARA BioSciences, Inc., a Delaware corporation (the
"Company").

                                    RECITALS

        A. It is expected that another company or other entity may from time to
time consider the possibility of acquiring the Company or that a change in
control may otherwise occur, with or without the approval of the Company's Board
of Directors (the "Board"). The Board recognizes that such consideration can be
a distraction to the Employee, an executive officer of the Company, and can
cause the Employee to consider alternative employment opportunities. The Board
has determined that it is in the best interests of the Company and its
stockholders to assure that the Company will have the continued dedication and
objectivity of the Employee, notwithstanding the possibility, threat or
occurrence of a Change of Control (as defined below) of the Company.

        B. The Board believes that it is in the best interests of the Company
and its stockholders to provide the Employee with an incentive to continue his
or her employment with the Company.

        C. The Board believes that it is imperative to provide the Employee with
certain benefits upon termination of the Employee's employment in connection
with a Change of Control, which benefits are intended to provide the Employee
with sufficient security to encourage the Employee to remain with the Company
notwithstanding the possibility of a Change of Control.

        D. To accomplish the foregoing objectives, the Board of Directors has
directed the Company, upon execution of this Agreement by the Employee, to agree
to the terms provided in this Agreement.

        E. Certain capitalized terms used in the Agreement are defined in
Section 3 below.

        In consideration of the mutual covenants contained in this Agreement,
and in consideration of the continuing employment of Employee by the Company,
the parties agree as follows:

               1. At-Will Employment. The Company and the Employee acknowledge
that the Employee's employment is and shall continue to be at-will, as defined
under applicable law. If the Employee's employment terminates for any reason,
including (without limitation) any termination prior to a Change of Control, the
Employee shall not be entitled to any payments or benefits, other than as
provided by this Agreement, or as may otherwise be available in accordance with
the terms of the Company's then existing employee plans and written policies in
effect at the time of termination. The terms of this Agreement shall terminate
upon the earlier of


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(i) the date on which Employee ceases to be employed as an executive officer of
the Company, other than as a result of an involuntary termination by the Company
without Cause (ii) the date that all obligations of the parties hereunder have
been satisfied, or (iii) one (1) year after a Change of Control. A termination
of the terms of this Agreement pursuant to the preceding sentence shall be
effective for all purposes, except that such termination shall not affect the
payment or provision of compensation or benefits on account of a termination of
employment occurring prior to the termination of the terms of this Agreement.

               2. Stock Options. Subject to Sections 4 and 5 below, in the event
of an Involuntary Termination of Employee's employment with the Company within
one year following a Change of Control, each stock option granted for the
Company's securities held by the Employee shall become fully vested and
immediately exercisable on the effective date of the Involuntary Termination and
shall be exercisable to the extent so vested in accordance with the provisions
of the stock option agreement and stock option plan pursuant to which such stock
option was granted.

               3. Definition of Terms. The following terms referred to in this
Agreement shall have the following meanings:

                      (a) Change of Control. "Change of Control" shall mean the
occurrence of any of the following events:

                             (i) Ownership. Any "Person" (as such term is used
in Sections 13(d) and 14(d) of the Securities Exchange Act of 1934, as amended)
is or becomes the "Beneficial Owner" (as defined in Rule 13d-3 under said Act),
directly or indirectly, of securities of the Company representing twenty percent
(20%) or more of the total voting power represented by the Company's then
outstanding voting securities without the approval of the Board of Directors of
the Company; or

                             (ii) Merger/Sale of Assets. A merger or
consolidation of the Company whether or not approved by the Board of Directors
of the Company, other than a merger or consolidation which would result in the
voting securities of the Company outstanding immediately prior thereto
continuing to represent (either by remaining outstanding or by being converted
into voting securities of the surviving entity) at least fifty percent (50%) of
the total voting power represented by the voting securities of the Company or
such surviving entity outstanding immediately after such merger or
consolidation, or the stockholders of the Company approve a plan of complete
liquidation of the Company or an agreement for the sale or disposition by the
Company of all or substantially all of the Company's assets.

                             (iii) Change in Board Composition. A change in the
composition of the Board of Directors of the Company, as a result of which fewer
than a majority of the directors are Incumbent Directors. "Incumbent Directors"
shall mean directors who either (A) are directors of the Company as of January
1, 2000 or (B) are elected, or nominated for election, to the Board of Directors
of the Company with the affirmative votes of at least a majority of the
Incumbent Directors at the time of such election or nomination (but shall not
include an individual



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whose election or nomination is in connection with an actual or threatened proxy
contest relating to the election of directors to the Company).

                      (b) Cause. "Cause" shall mean (i) gross negligence or
willful misconduct in the performance of the Employee's duties to the Company
where such gross negligence or willful misconduct has resulted or is likely to
result in substantial and material damage to the Company or its subsidiaries,
(ii) repeated unexplained or unjustified absence from the Company, (iii) a
material and willful violation of any federal or state law; (iv) commission of
any act of fraud with respect to the Company; or (v) conviction of a felony or a
crime involving moral turpitude causing material harm to the standing and
reputation of the Company, in each case as determined in good faith by the Board
of Directors of the Company.

                      (c) Involuntary Termination. "Involuntary Termination"
shall include any termination by the Company other than for Cause and the
Employee's voluntary termination, upon 30 days prior written notice to the
Company, following (i) any reduction of the Employee's base compensation (other
than in connection with a general decrease in base salaries for most similarly
situated employees of the successor corporation); or (ii) the Employee's refusal
to relocate to a location more than 50 miles from the Company's current
location.

               4. Limitation-on Payments. To the extent that any of the payments
or benefits provided for in this Agreement to the Employee constitute "parachute
payments" within the meaning of Section 280G of the Internal Revenue Code of
1986, as amended (the "Code") and, but for this Section 5, would be subject to
the excise tax imposed by Section 4999 of the Code, the Company shall reduce the
aggregate amount of such payments and benefits such that the present value
thereof (as determined under the Code and the applicable regulations) is equal
to 2.99 times the Employee's "base amount" as defined in Section 28OG(b)(3) of
the Code.

               5. Certain Business Combinations. In the event it is determined
by the Board, upon consultation with Company management and the Company's
independent auditors, that the enforcement of any Section of this Agreement,
including, but not limited to, Section 2 hereof, which allows for the
acceleration-of-vesting of option shares upon the effective date of an
Involuntary Termination within one year following a Change of Control, would
preclude accounting for any proposed business combination of the Company
involving a Change of Control as a pooling of interests, and the Board otherwise
desires to approve such a proposed business transaction which requires as a
condition to the closing of such transaction that it be accounted for as a
pooling of interests, then any such Section of this Agreement shall be null and
void. For purposes of this Section 5, the Board's determination shall require
the unanimous approval of the non-employee Board members.

               6. Successors. Any successor to the Company (whether direct or
indirect and whether by purchase, lease, merger, consolidation, liquidation or
otherwise) to all or substantially all of the Company's business and/or assets
shall assume the obligations under this Agreement and agree expressly to perform
the obligations under this Agreement in the same manner and to the same extent
as the Company would be required to perform such obligations in the absence of a
succession. The terms of this Agreement and all of the Employee's rights
hereunder shall inure to



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the benefit of, and be enforceable by, the Employee's personal or legal
representatives, executors, administrators, successors, heirs, distributees,
devisees and legatees.

               7. Notice. Notices and all other communications contemplated by
this Agreement shall be in writing and shall be deemed to have been duly given
when personally delivered or when mailed by U.S. registered or certified mail,
return receipt requested and postage prepaid. Mailed notices to the Employee
shall be addressed to the Employee at the home address which the Employee most
recently communicated to the Company in writing. In the case of the Company,
mailed notices shall be addressed to its corporate headquarters, and all notices
shall be directed to the attention of its Secretary.

               8. Miscellaneous Provisions.

                      (a) No Duty to Mitigate. The Employee shall not be
required to mitigate the amount of any payment contemplated by this Agreement
(whether by seeking new employment or in any other manner), nor, except as
otherwise provided in this Agreement, shall any such payment be reduced by any
earnings that the Employee may receive from any other source.

                      (b) Waiver. No provision of this Agreement shall be
modified, waived or discharged unless the modification, waiver or discharge is
agreed to in writing and signed by the Employee and by an authorized officer of
the Company (other than the Employee). No waiver by either party of any breach
of, or of compliance with, any condition or provision of this Agreement by the
other party shall be considered a waiver of any other condition or provision or
of the same condition or provision at another time.

                      (c) Whole Agreement. No agreements, representations or
understandings (whether oral or written and whether express or implied) which
are not expressly set forth in this Agreement have been made or entered into by
either party with respect to the subject matter hereof. This Agreement
supersedes any agreement of the same title and concerning similar subject matter
dated prior to the date of this Agreement, and by execution of this Agreement
both parties agree that any such predecessor agreement shall be deemed null and
void; provided, however, that the parties acknowledge that (i) the Company and
Employee are parties to a prior agreement that provides certain
acceleration-of-vesting benefits to Employee in the event of a change of control
(the "Prior Agreement"), and (ii) the terms of the Prior Agreement shall be of
full force and effect in the event that the provisions of Section 5 hereof
prevent the Employee from receiving the benefits set forth in Section 2 hereof.

                      (d) Choice of Law. The validity, interpretation,
construction and performance of this Agreement shall be governed by the laws of
the State of California without reference to conflict of laws provisions.

                      (e) Severability. If any term or provision of this
Agreement or the application thereof to any circumstance shall, in any
jurisdiction and to any extent, be invalid or unenforceable, such term or
provision shall be ineffective as to such jurisdiction to the extent of



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such invalidity or unenforceability without invalidating or rendering
unenforceable the remaining terms and provisions of this Agreement or the
application of such terms and provisions to circumstances other than those as to
which it is held invalid or unenforceable, and a suitable and equitable term or
provision shall be substituted therefor to carry out, insofar as may be valid
and enforceable, the intent and purpose of the invalid or unenforceable term or
provision.

                      (f) Arbitration. Any dispute or controversy arising under
or in connection with this Agreement may be settled at the option of either
party by binding arbitration in the County of Santa Clara, California, in
accordance with the rules of the American Arbitration Association then in
effect. Judgment may be entered on the arbitrator's award in any court having
jurisdiction. Punitive damages shall not be awarded.

                      (g) Legal Fees and Expenses. The parties shall each bear
their own expenses, legal fees and other fees incurred in connection with this
Agreement.

                      (h) No Assignment of Benefits. The rights of any person to
payments or benefits under this Agreement shall not be made subject to option or
assignment, either by voluntary or involuntary assignment or by operation of
law, including (without limitation) bankruptcy, garnishment, attachment or other
creditor's process, and any action in violation of this subsection (h) shall be
void.

                      (i) Employment Taxes. All payments made pursuant to this
Agreement will be subject to withholding of applicable income and employment
taxes.

                      (j) Assignment by Company. The Company may assign its
rights under this Agreement to an affiliate, and an affiliate may assign its
rights under this Agreement to another affiliate of the Company or to the
Company; provided, however, that no assignment shall be made if the net worth of
the assignee is less than the net worth of the Company at the time of
assignment. In the case of any such assignment, the term "Company" when used in
a section of this Agreement shall mean the corporation that actually employs the
Employee.

                      (k) Counterparts. This Agreement may be executed in
counterparts, each of which shall be deemed an original, but all of which
together will constitute one and the same instrument.

        IN WITNESS WHEREOF, each of the parties has executed this Agreement, in
the case of the Company by its duly authorized officer, as of the day and year
first above written.

ACLARA BIOSCIENCES, INC.                     EMPLOYEE


By: /s/ WENDY R. HITCHCOCK                   /s/ JOSEPH M. LIMBER
    -------------------------------          -----------------------------------

Title: Secretary                             Joseph M. Limber
       ----------------------------          -----------------------------------
                                             Printed Name



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