EXHIBIT 10.19 (C)



                           LOAN MODIFICATION AGREEMENT

     This Loan Modification  Agreement is entered Into as of August 15, 1997, by
and between Ventana Medical  Systems,  Inc.  ("Borrower")  whose address Is 3865
North Business Drive,  Tucson,  Arizona 85705,  and Silicon Valley Bank ("Bank")
whose address is 3003 Tasman Drive, Santa Clara, CA 95054.

1. DESCRIPTION OF EXISTING  INDEBTEDNESS:  Among other Indebtedness which may be
owing by Borrower to Bank, Borrower is Indebted to Bank pursuant to, among other
documents,  a Loan and Security  Agreement,  dated February 20, 1995, (the "Loan
Agreement),  as amended.  The Loan Agreement provided for, among other things, a
Committed  Line In the original  principal  amount of Two Million  Seven Hundred
Fifty Thousand and 001100 Dollars  ($2,750,000.00)  (the "Revolving  Facility").
Defined terms used but not otherwise defined herein shall have the same meanings
as in the Loan Agreement

Hereinafter,  all indebtedness owing by Borrower to Bank shall be referred to as
the "Obligations".

2.  DESCRIPTION OF COLLATERAL AND  GUARANTIES.  Repayment of the  Obligations is
secured by the  Collateral  as defined in the Loan  Agreement,  and a Collateral
Assignment,  Patent  Mortgage and Security  Agreement  dated  February 20, 1995.
Additionally,  repayment of the  Obligations Is guaranteed by Bio Tek Solutions,
Inc. (the "Guarantor")  pursuant to an Unconditional  Guaranty (the "Guaranty").
The Guaranty Is secured by a Guarantor  Security Agreement dated March 22, 1996.
Concurrently  herewith,  the  aforementioned  Guaranty  and  Guarantor  Security
Agreement are being released  pursuant to  the terms of  this Loan  Modification
Agreement

Hereinafter,  the  above-described  security documents and guaranties,  together
with all other documents securing repayment of the Obligations shall be referred
to as the "Security Documents".  Hereinafter,  the Security Documents,  together
with all  other  documents  evidencing  or  securing  the  Obligations  shall be
referred to as the "Existing Loan Documents".

3.   DESCRIPTION OF CHANGE IN TERMS.

     A.   Modification(s) to Loan Agreement.

          1.   Section 1.1 entitled  "Definitions'  is hereby amended in part to
               read as follows:

               "Revolving Maturity Date" means February 15,1998.

          2.   Subsection (a) of Section 2.3 entitled "Interest Rates,  Payments
               and  Calculations"  is hereby amended in its entirety) to read as
               follows:

               Any Advances  evidenced by the Note shall bear  Interest,  on the
               average  Daily  Balance,  at a rate per annum  equal to the Prime
               Rate.

          3.   Section  6.8  entitled  "Quick  Ratio" is hereby  amended  in its
               entirety to read as follows:

               Borrower  shall  maintain,  on  a  quarterly  basis  and  without
               consolidation,  a ratio of Quick Assets to Current Liabilities of
               at least 1.50 to 1.00.

          4.   Section 6.10 entitled "Debt-Net Worth Ratio" is hereby amended in
               its entirety to read as follows:

               Borrower  shall  maintain,  on  a  quarterly  basis  and  without
               consolidation a ratio of Total Liabilities to Tangible Net Worth
               of not more than 1.00 to 1.00.


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     B.   Release of Guaranty.

               Bank, by Its acceptance hereof, agrees to release the Guaranty of
               Bio Tek  Solutions,  Inc.,  provided that no Event of Default has
               occurred  and  is  continuing  under  any of  the  Existing  Loan
               Documents (other than the default waived herein).  All parties to
               this Loan  Modification  Agreement  acknowledge  and  agree  that
               Bank's  release of the  Guaranty  In no way shall limit or impair
               Bank's right against  Borrower or against any security  pledge by
               the foregoing parties.

     C.   Waiver of Default.

               Bank hereby  waives  Borrowers  existing  default  under the Loan
               Agreement  by virtue of  Borrower's  failure  to comply  with the
               profitability  covenant  as of the quarter  ended June 30,  1997.
               Bank's  waiver of Borrower's  compliance  of this covenant  shall
               apply only to the foregoing period. Accordingly,  for the quarter
               ending  September  30,1997,  Borrower shall be in compliance with
               this covenant

               Bank's agreement to waive the  above-described  default (1) in no
               way shall be deemed an agreement by the Bank to waive  Borrower's
               compliance  with the  above-described  covenant  as of all  other
               dates  and (2) shall not  limit or  Impair  the  Bank's  right to
               demand strict  performance of this covenant as of all other dates
               and (3)  shall not limit or  impair  the  Bank's  right to demand
               strict performance of all other covenants as of any date.

4. CONSISTENT  CHANGES.  The Existing Loan Documents are hereby amended wherever
necessary to reflect the changes described above.

5. NO DEFENSES OF BORROWER.  Borrower (and each  guarantor  and pledgor  signing
below).  agrees  that it has no  defenses  against  the  obligations  to pay any
amounts under the Existing Loan Documents.

6. CONTINUING VALIDITY.  Borrower (and each guarantor and pledgor signing below)
understands  and agrees that in  modifying  the  existing  Obligations,  Bank is
relying upon Borrowers representations, warranties, and agreements, as set forth
in the Existing Loan Documents.  Except as expressly  modified  pursuant to this
Loan  Modification  Agreement,  the terms of the Existing Loan Documents  remain
unchanged and In full force and effect Bank's  agreement to modifications to the
existing  Obligations  pursuant to this Loan  Modification  Agreement  In no way
shall obligate Bank to make any future modifications to the Obligations. Nothing
In this Loan  Modification  Agreement  shall  constitute a  satisfaction  of the
Obligations.  It is the  Intention  of Bank and  Borrower  to  retain  as liable
parties all makers and endorsers of Existing Loan Documents, unless the party is
expressly released by sank In writing. No maker,  endorsers or guarantor will be
released  by  virtue  of this  Loan  Modification  Agreement  The  terms of this
paragraph apply not only to this Loan  Modification  Agreement,  but also to all
subsequent loan modification agreements.



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     This Loan  Modification  Agreement Is executed as of the date first written
above.

BORROWER:                                   BANK:

VENTANA MEDICAL SYSTEMS, INC.               SILICON VALLEY BANK


By: /s/ Pierre Sice                         By: /s/ Kevin J. Conway
   -------------------------------              --------------------------------
 Name: Pierre Sice                          Name: Kevin J. Conway
Title: VP & CFO                             Title: VP














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