Document:

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                                                                     EXHIBIT 4.1

                                   AGREEMENT

        Pursuant to Item 601(b)(4)(iii)(a) of Regulation S-K promulgated under
the Securities Exchange Act of 1934, PETRO STOPPING CENTERS, L.P. ("PSC") and
PETRO STOPPING CENTERS HOLDINGS, L.P. ("Holdings") hereby agree to furnish a
copy of each of the following documents to the Securities and Exchange
Commission (the "Commission") promptly upon the Commission's request:

        1.  Indenture dated as of May 24, 1994 between Petro PSC, L.P. as
            Guarantor and First Trust National Association, as Trustee;

        2.  Specimen of 12 1/2% Senior Notes due 2002;

        3.  Guarantee Agreement dated as of May 24, 1994 between Petro PSC, L.P.
            as Guarantor, and First Trust National Association, as Trustee;

        4.  Warrant Agreement dated as of May 24, 1994 among Petro Stopping
            Centers, L.P., Petro Financial Corporation and First Trust National
            Association; and

        5.  Specimen of Exchangeable Debt Warrant;

        PSC and Holdings hereby represent to the Commission that the total
amount owed by PSC pursuant to the 12 1/2% Senior Notes due 2002 does not exceed
10% of the total assets of PCS or Holdings and their subsidiaries on a
consolidated basis, in each case as of December 31, 1999 and as the date hereof.

DATED:  March 29, 2000

                              PETRO STOPPING CENTERS HOLDINGS, L.P.

                              By:     /s/ David A. Appleby
                                  ----------------------------------------
                              Name:       David A. Appleby
                                    --------------------------------------
                              Title:  Vice President Finance and Treasurer
                                    --------------------------------------

                              PETRO STOPPING CENTERS, L.P.

                              By:     /s/ David A. Appleby
                                  ----------------------------------------
                              Name:       David A. Appleby
                                    --------------------------------------
                              Title:  Vice President Finance and Treasurer
                                    --------------------------------------<PAGE>

                                                                   Exhibit 10.23

                                   AGREEMENT

        Pursuant to Item 601(b)(4)(iii)(a) of Regulation S-K promulgated under
the Securities Exchange Act of 1934, PETRO STOPPING CENTERS, L.P. ("PSC") and
PETRO STOPPING CENTERS HOLDINGS, L.P. ("Holdings") hereby agree to furnish a
copy of each of the following documents to the Securities and Exchange
Commission (the "Commission") promptly upon the Commission's request:

        1.      Indenture dated as of May 24, 1994 between Petro PSC, L.P. as
                Guarantor and First Trust National Association, as Trustee;

        2.      Specimen of 12-1/2% Senior Notes due 2002;

        3.      Guarantee Agreement dated as of May 24, 1994 between Petro PSC,
                L.P. as Guarantor, and First Trust National Association as
                Trustee;

        4.      Warrant Agreement dated as of May 24, 1994 among Petro Stopping
                Centers, L.P., Petro Financial Corporation and First Trust
                National Association; and

        5.      Specimen of Exchangeable Debt Warrant;

        PSC and Holdings hereby represent to the Commission that the total
amount owed by PSC pursuant to the 12 1/2% Senior Notes due 2002 does not exceed
10% of the total assets of PCS or Holdings and their subsidiaries on a
consolidated basis, in each case as of December 31, 1999 and as the date hereof.

DATED:  March 29, 2000

                                PETRO STOPPING CENTERS HOLDINGS, L.P.

                                By:  /s/ DAVID A. APPLEBY
                                   ------------------------------------------
                                Name:    David A. Appleby
                                     ----------------------------------------
                                Title: Vice President Finance and Treasurer
                                      ---------------------------------------

                                PETRO STOPPING CENTERS, L.P.

                                By:  /s/ DAVID A. APPLEBY
                                   ------------------------------------------
                                Name:    David A. Appleby
                                     ----------------------------------------
                                Title: Vice President Finance and Treasurer
                                      ---------------------------------------<PAGE>

                                                                    EXHIBIT 10.8

     Confidential Materials omitted and filed separately with the Securities and
Exchange Commission. Asterisks denote omissions.

                 AMENDMENT TO DEVELOPMENT AND SUPPLY AGREEMENT

     This Amendment to Development and Supply Agreement ("Amendment"), effective
as of January 13, 2000 (the "Amendment Effective Date"), by and between Luminex
Corporation, a Delaware corporation with principal offices at 12212 Technology
Boulevard, Austin, Texas 79727 ("LUMINEX") and Bio-Rad Laboratories, Inc., a
Delaware corporation with principal offices at 1000 Alfired Nobel Drive,
Hercules, California 94547 ("BIO-RAD") (LUMINEX and BIO-RAD collectively, the
"Parties"), amends that certain Development and Supply Agreement by and between
the Parties effective as of March 19, 1999 (the "Agreement").

     WHEREAS, the Parties desire to amend the Agreement as set forth herein
below.

     NOW, THEREFORE, the Parties agree as follows:

     AMENDMENT. This Amendment hereby amends the Agreement to incorporate the
terms and conditions set forth in this Amendment. The relationship of the
Parties shall continue to be governed by the terms and conditions of the
Agreement, as amended herein; and in the event that there is any conflict
between the terms and conditions of the Agreement and this Amendment, the term
and conditions of this Amendment shall control. As used in this Amendment, all
capitalized terms shall have the meanings defined for such terms in this
Amendment or, if not defined in the Amendment, the meanings defined in the
Agreement

MODIFICATIONS TO THE AGREEMENT.

I.  Exhibit A to the Agreement is hereby amended to add the following:

    [**]

2.       "Field Two" in Exhibit A of the Agreement is deleted and replaced with
         the following: [**]

3.       Section 1.7 of the Agreement is hereby amended to add the following:

         "Fields  Five and Six, as set forth in Exhibit A, shall be limited to
         Research Use.

4.       The Agreement is hereby amended to add the following Section 1.23:

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[**] Indicates that material has been omitted and confidential treatment
requested therefor. All such material has been filed separately with the
Commission pursuant to Rule 406.
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         "1.23  "Research  Use" means the  performance  of basic life science
                 -------------
         research,  including high  throughput  screening,  but in all events
         excluding   laboratory   testing  for  human   clinical   diagnostic
         purposes."

  5.     Section 2.1 (d) of the Agreement is hereby amended to delete the last
         sentence of such section and replace it with the following:

         "Notwithstanding any provision of this Agreement to the contrary,
         including the provisions of Section 8.3, if the parties do not agree by
         June 1, 2000 upon: (i) a tolling fee for Magnetic Beads of less than or
         equal to [**] ($[**]) for every three thousand (3000) units of Magnetic
         Beads purchased, where a "unit" of Magnetic Beads is equal to one
         microsphere bead, and (ii) the development of a beadmap for Magnetic
         Beads which shall be capable of discriminating between [**] analytes
         with a minimum accuracy level of [**] percent ([**]%) and no more than
         [**] percent ([**]%) misclassification, all rights of BIO-RAD to have
         LUMINEX treat the Magnetic Beads hereunder and all obligations of
         LUMINEX with respect to the treatment or processing of Magnetic Beads
         for BIO-RAD hereunder shall automatically terminate unless otherwise
         agreed in writing, provided, however, that in such event BIO-RAD shall
                            --------  -------
         have the right to have the Magnetic Beads processed by BIO-RAD or a
         third party and, for the avoidance of doubt, BIO-RAD shall retain the
         exclusivity rights with respect to Magnetic Beads set forth in Section
         3.2 of this Agreement."

  6.     Section 3.1 of the Agreement is hereby amended to add the following
         after the phrase "only as part of BIO-RAD Systems":

         "'or, within Fields Five and Six, Luminex100 Systems separately"

  7.     Section 3.1 of the Agreement is hereby further amended to add the
         following after the phrase "BIO-RAD U.S. Diagnostics Division
         management":

         "or BIO-RAD U.S. Life Sciences Division management"

  8.     Section 3.1 of the Agreement is hereby further amended to add the
         following after the phrase "in conjunction with their operation of BIO-
         RAD Systems":

         "and Luminex100 Systems"

  9.     Section 3.3(b)(i) of the Agreement is hereby amended to add the
         following after the phrase "human clinical diagnostics industry":

         "or the life science research industry"

  10.    Section 3.5 of the Agreement is hereby amended to add the following
         after the phrase "BIO-RAD U.S. Diagnostics Division management":

         "or BIO-RAD U.S. Life Sciences Division management"

         ------------
         [**] Indicates that material has been omitted and confidential
         treatment requested therefor. All such material has been filed
         separately with the Commission pursuant to Rule 406.

                                      -2-
<PAGE>

  11.     Section 4.6(c) of the Agreement is hereby amended to add the following
          after the phrase "in vitro diagnostic Kits":
                            --------

          "or Kits for Research Use"

  12.     Section 4.8 of the Agreement is hereby amended to add the following
          after the phrase "All invoices shall be sent to the following
          address:"

          "For  Clinical  Diagnostics  Group,  Bio-Rad  Laboratories,   Accounts
          Payable, 4000 Alfred Nobel Drive,  Hercules,  California 49547, or for
          Life Sciences Group,  Bio Rad  Laboratories,  Accounts  Payable,  2000
          Alfred Nobel Drive, Hercules, California."

  13.     Section 4.11 (a) of the Agreement is hereby amended to add the
          following after the phrase "At the Effective Date:"

          "and at the Amendment Effective Date"

  14.     Section 4.11(b) of the Agreement is hereby amended to add the
          following after the phrase "the date of this Agreement:"

          "and at the Amendment Effective Date"

  15.     Section 5.2 of the Agreement is hereby amended to read in its entirety
          as follows:

          "Exclusivity of Efforts.  To avoid conflicts of interest,  BIO-RAD and
           ----------------------
          its Affiliates will not manufacture, market, sell or otherwise
          distribute any materials, technologies or products, other than
          Luminex100 Systems, Bio-Rad Systems and Kits, specifically designed
          for performing Multiplexed Bead Assays using flow cytometry based
          detection of particles for use in the human clinical diagnostics
          industry or for Research Use within the Fields. In addition, BIO-RAD
          agrees that except for performing its development obligations as
          agreed pursuant to Section 2.1 and the development and manufacture of
          Bio-Rad Systems, neither BIO-RAD's clinical diagnostics division or
          group nor BIO-RAD's life sciences division or group will directly or
          indirectly develop any materials, technologies or products
          specifically designed for performing Multiplexed Bead Assays within
          the Fields using flow cytometry based detection of particles. BIO-RAD
          further agrees that it will not provide research and development
          funding to or license any third party to develop, manufacture, market,
          sell or otherwise distribute any materials, technologies or products
          specifically designed for performing Multiplexed Bead Assays within
          the Fields using flow cytometry based detection of particles. It is
          understood that the foregoing sentence shall not apply to passive
          investments by BIO-RAD in equity of third parties for use in the human
          clinical diagnostics industry or for Research Use."

                                      -3-
<PAGE>

  16.     Section 8.1 of the Agreement is hereby amended to add the following
          after the words "each such renewal term":

          "but which in no case shall vary by more than [**] percent ([**]%)
          from the average of the three prior years' sales. Notwithstanding the
          foregoing provisions of this Section 8.1, Fields Five and Six shall be
          included in the Agreement during each of the renewal terms only if the
          parties agree upon and BIO-RAD meets annual sales goals with respect
          to Kits in Fields Five and Six, which annual sales goals shall be
          negotiated in good faith by the parties prior to the commencement of
          each such renewal term but which in no case shall vary by more than
          [**] percent ([**]%) from the average of the three prior years'
          sales."

  17.     Section 8.3 of the Agreement is hereby amended to add the following
          after the phrase "Luminex100 System" in the second line of that
          Section:

          "(or,  with  respect to Fields other than Fields One,  Two,  Three or
          Four, beginning five (5) years after the addition to the Agreement of
          such Field or Fields)"

  18.     Section 8.3 of the Agreement is hereby further amended to delete the
          last sentence thereof and replace if with the following:

          "If BIO-RAD has not made commercially available Kits incorporating
          Magnetic Beads in each of Fields One, Two, Three and Four within three
          (3) years of the Effective Date plus any extension of time necessary
          to produce Magnetic Beads reasonably acceptable to BIO-RAD, LUMINEX
          may terminate BIO-RAD's exclusive rights with respect to Magnetic
          Beads in Fields One, Two, Three and Four by written notice to BIO-RAD.
          If BIO-RAD has not made commercially available Kits incorporating
          Magnetic Beads in each of Fields Five and Six within three (3) years
          of the Amendment Effective Date plus any extension of time necessary
          to produce Magnetic Beads reasonably acceptable to BIO-RAD, LUMINEX
          may terminate BIO-RAD's exclusive rights with respect to Magnetic
          Beads in Five and Six by written notice to BIO- RAD.

  19.     Section 12.2 of the Agreement is hereby amended to add the following
          after the word "Group" in the third line of that Section:

          "or BIO-RAD Vice-President Life Sciences Division, as appropriate,"

  20.     Section 12.8 of the Agreement is hereby amended to add the following
          at the end of that Section:

          "with a further copy to:
          Bio-Rad Laboratories, Inc. Life Science Group
          2000 Alfred Nobel Drive
          Hercules, California 94547
          Attention: Vice-President, Life Science Group"

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[**]   Indicates that material has been omitted and confidential treatment
requested therefor. All such material has been filed separately with the
Commission pursuant to Rule 406.

                                      -4-

<PAGE>

      Fax: (510) 741-5803"

 21.  ADDITIONAL TERM: ADVANCE AGAINST ROYALTIES. Upon the effective date of
      this Amendment, BIO-RAD shall pay to LUMINEX an aggregate of [**] Dollars
      ($[**]) for both of Fields Five and Six as an Advance.

 22.  The additional fee for expansion of Field Two set forth in Exhibit A of
      the Agreement is waived.

 ENTIRE AGREEMENT.  Notwithstanding  the first sentence of Section 12.17 of the
    Agreement, together the Agreement (including the Exhibits thereto) and this
    Amendment constitute the entire agreement between the Parties in connection
    with the subject matter thereof and supersede all prior and contemporaneous
    agreements, understandings, negotiations and discussions, whether oral or
    written, of the Parties.

      IN WITNESS WHEREOF, the Parties have executed this Amendment.

  LUMINEX CORPORATION                                 BIO-RAD LABORATORIES, INC.

  By:    /s/ Randel S. Marfin                        By:    /s/ Sanford Wadler
        ----------------------                              --------------------
  Name:  Randel S. Marfin                            Name:  Sanford Wadler
        ----------------------                              --------------------
  Title: Vice President - Business Development       Title: Vice President
        --------------------------------------              --------------------

----------
[**] Indicates that material has been omitted and confidential treatment
requested therefor. All such material has been filed separately with the
Commission pursuant to Rule 406.

                                      -5-

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