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                                                                  EXHIBIT 10.17



                                 THIRD AMENDMENT
                                       TO
                    CONVERTIBLE DEBENTURE PURCHASE AGREEMENT

           THIS THIRD AMENDMENT TO CONVERTIBLE DEBENTURE PURCHASE AGREEMENT (the
"Amendment") is entered into as of December 31, 1997, by and between DISC, Inc.,
a California corporation (the "Company"), and MK GVD FUND (the "Purchaser").

                                R E C I T A L S :

           A. WHEREAS on March 29, 1996 the Company and Purchaser entered into a
Convertible Debenture Purchase Agreement pursuant to which the Company agreed to
sell, and Purchaser agreed to purchase, an aggregate of $1,400,000 in principal
amount of Convertible Debentures, each convertible into shares of the Company's
Preferred Stock, which Agreement was amended as of December 31, 1996 to increase
the aggregate amount of Convertible Debentures to be purchased thereunder to
$3,400,000 and was amended as of April 11, 1997 to increase the aggregate amount
of Convertible Debenture to be purchased thereunder to $4,430,000.

           B. The Company and Optionee now seek to amend the Agreement to
increase the total amount of Convertible Debentures which Purchaser agrees to
purchase thereunder.

           NOW, THEREFORE, for good and valuable consideration, receipt of which
is hereby acknowledged, and in consideration of the mutual covenants set forth
herein, the parties hereto agree as follows:

           1. DEFINITIONS. Unless otherwise defined herein, capitalized terms
used in this Amendment shall have the same meanings ascribed to them in the
Convertible Debenture Purchase Agreement.

           2. AMENDMENT TO CONVERTIBLE DEBENTURE PURCHASE AGREEMENT. Section
1.1(a) of the Convertible Debenture Purchase Agreement is hereby amended to
provide that Purchase agrees to purchase, and the Company agrees to issue and
sell, an aggregate of $4,730,000 in principal amount of Convertible Debentures.

           3. ENTIRE AGREEMENT; AMENDMENT. The Convertible Debenture Purchase
Agreement, as amended by this Amendment, constitutes the full and complete
agreement and understanding between the parties hereto regarding the subject
matter of the Convertible Debenture Purchase Agreement and shall supersede all
prior communications, representations, understandings or agreements, if any,
whether oral or written, concerning the subject matter contained in the
Convertible Debenture Purchase Agreement, as so amended, and that no provision
of the Convertible Debenture Purchase Agreement, as so amended, may be modified,
amended, waived or discharged, in whole or in part, except in accordance with
its terms.

           4. FORCE AND EFFECT. Except as modified by this Amendment, the terms
and provisions of the Convertible Debenture Purchase Agreement are hereby
ratified and confirmed and are and shall remain in full force and effect. Should
any inconsistency arise between this Amendment and the Convertible Debenture
Purchase Agreement as to the specific matters which are the subject of this
Amendment, the terms and conditions of this Amendment shall control. This
Amendment shall be construed to be part of the Convertible Debenture Purchase
Agreement and shall be deemed incorporated into the Convertible Debenture
Purchase Agreement by this reference.



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           5. COUNTERPARTS. This Amendment may be executed in one or more
counterparts, each of which shall be an original but all of which together shall
constitute one instrument.


           IN WITNESS WHEREOF, the Company has caused this Amendment to be
executed in duplicate on its behalf by its duly authorized officer and Optionee
has also executed this Amendment in duplicate, all as of the day and year
indicated above.

                                              DISC, INC.
                                              a California corporation


                                              By:   /s/ Henry Madrid
                                                 ------------------------------
                                                    Henry Madrid,
                                                    Vice President and
                                                    Chief Financial Officer


                                              PURCHASER:

                                              MK GVD FUND


                                              By:   /s/ Michael D. Kaufman
                                                 ------------------------------
                                                    Michael D. Kaufman,
                                                    General Partner






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