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                                                                   EXHIBIT 10.12

                                 SIDE AGREEMENT

    This Side Agreement ("Agreement") is dated as of the 19th day of March 1999,
is by and between ACLARA BIOSCIENCES, INC., 3906 Trust Way, Hayward, California
94545-3716 ("ACLARA") and THE PERKIN-ELMER CORPORATION, having its PERKIN-ELMER
Biosystems Division at 850 Lincoln Centre Drive, Foster City, CA 94404
("PERKIN-ELMER").

A.  WHEREAS, ACLARA and PERKIN-ELMER (the "Parties") will enter into a
    collaboration agreement ("Collaboration Agreement") at a date coincident
    with the execution of this Agreement;

B.  WHEREAS, the Parties will enter into a Custom Instrument Development and
    Commercialization Agreement ("PRI Agreement") with the R.W. Johnson
    Pharmaceutical Research Institute, a Division of Ortho-McNeil
    Pharmaceutical, Inc. (PRI), at a date coincident with the execution of this
    Agreement, for the purpose of developing certain HTS Systems, under which
    the Parties will receive [*] from PRI;

C.  WHEREAS, PERKIN-ELMER has already received [*] from PRI;

D.  WHEREAS, the Parties desire to clarify certain matters relating to the
    Collaboration Agreement as it pertains to their participation in the PRI
    Agreement.

NOW THEREFORE, in considerations of the mutual covenants and promises contained
in this Agreement, the Parties agree as follows:

1.  Definitions. As used herein, capitalized terms will have the meaning
    ascribed to them in the PRI Agreement or the Collaboration Agreement.

2.  Distribution of Funds Received from PRI. Under the PRI Agreement, PRI will
    provide the Parties with [*] with which to undertake the development of
    certain HTS Systems for PRI, the money to be distributed between the Parties
    as follows: [*] will go to PERKIN-ELMER, and [*] will go to ACLARA. In
    particular, PERKIN-ELMER will pay ACLARA, within three (3) days after
    received by PERKIN-ELMER, [*] scheduled to be received from PRI pursuant to
    Section 6.1 of the PRI Agreement, and [*] of funds received from PRI
    pursuant to Sections 6.2, 6.3 and 6.4 of the PRI Agreement. PERKIN-ELMER
    will have no liability to ACLARA on account of PRI's failure for any reason
    whatsoever to make any payment due PERKIN-ELMER, or PERKIN-ELMER and ACLARA,
    under the PRI

[*]  CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
     BRACKETS, HAS BEEN OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND
     EXCHANGE COMMISSION PURSUANT TO RULE 406 OF THE SECURITIES ACT OF 1933, AS
     AMENDED.


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    Agreement. PERKIN-ELMER's sole obligation will be to pay ACLARA the
    designated portion of whatever funds PERKIN-ELMER does in fact receive from
    PRI.

3.  Commercial Sale. Sales of Microfluidic Electrophoresis Devices to PRI under
    the PRI Agreement will not be considered commercial sales as that term is
    used in Section 3.3.8 of the Collaboration Agreement.

4.  Technology Access Partner. PRI is a Technology Access Partner and the PRI
    Agreement is a Technology Access Agreement.

5.  Royalties. Any royalty received from PRI for PRI's manufacture or sale of
    Microfluidic Electrophoresis Devices will be split [*] between PERKIN-ELMER
    and ACLARA.

6.  Installation, Service and Support. ACLARA and PERKIN-ELMER will determine
    as between themselves which company will provide installation, service and
    support to PRI pursuant to the provisions of Section 5.5 of the PRI
    Agreement. Generally, it is contemplated that ACLARA will provide support
    on matters involving Microfluidic Chips and assay chemistry, and
    PERKIN-ELMER will provide support on matters involving HTS Chip Devices and
    Commercial Instruments. Each party will bear its own expenses of providing
    installation, service and support to PRI.

7.  Allocation of Personnel. Commencing on the date the PRI Agreement is signed
    by the last of the parties thereto or as soon thereafter as is feasible, but
    in any event no later than ninety (90) days after such date, and during the
    remainder of the original [*] term of the Project, which [*] term ends
    [*], PERKIN-ELMER agrees to allocate an average of at least fourteen FTE's
    to the Project, and ACLARA agrees to allocate an average of at least [*]
    FTE's to the Project. In the event that the original [*] term of the Project
    is extended by PERKIN-ELMER and ACLARA on the other hand, or PRI on the
    other, PERKIN-ELMER will allocate and expend [*] of the number of FTE's
    allocated to the Project and ACLARA will allocate and expend [*] of the
    number of FTE's allocated and expended to the Project, unless the
    PERKIN-ELMER/ACLARA Joint Steering Committee otherwise agrees.

8.  Manufacturing Cost. Manufacturing Cost within the meaning of the
    Collaboration Agreement will never be more than Fully Burdened Manufacturing
    Cost and Fully Burdened Chip Manufacturing Cost within the meaning of the
    PRI Agreement, notwithstanding anything contained in the Collaboration
    Agreement to the contrary.

[Signature page Follows]

[*]  CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
     BRACKETS, HAS BEEN OMITTED AND FILED SEPARATELY WITH THE SECURITIES AND
     EXCHANGE COMMISSION PURSUANT TO RULE 406 OF THE SECURITIES ACT OF 1933, AS
     AMENDED.


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IN WITNESS WHEREOF, the Parties, through their authorized officers, have
executed this Agreement as of the date first written above.

ACLARA BIOSCIENCES, INC.             THE PERKIN-ELMER CORPORATION,
                                     THROUGH ITS PE BIOSYSTEMS
                                     DIVISION

By: /s/ JOSEPH M. LIMBER             By: /s/ MICHAEL HUNKAPILLER
   --------------------------            ------------------------------

Name: Joseph M. Limber               Name: Michael Hunkapiller
     ------------------------             -----------------------------

Title: President, CEO                Title: Executive Vice President
      -----------------------              ----------------------------

Date: March 19, 1999                 Date: March 19, 1999
     ------------------------             -----------------------------


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