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                            HEWLETT-PACKARD COMPANY

                                  June 6, 1999

BCC Acquisition II LLC
c/o Bay City Capital
750 Battery Street, Suite 600
San Francisco, California 94111
Attention: Fred Craves

     Re: Diametrics Medical, Inc.

Dear Mr. Craves:

     Reference is made to the Common Stock Purchase Agreement between Diametrics
Medical, Inc. and Hewlett-Packard Company to be entered into on the date hereof
substantially in the form attached hereto as Exhibit A (the "Purchase
Agreement"). All capitalized terms used but not otherwise defined in this letter
shall have the meanings ascribed to them in the Purchase Agreement.

     Based on our review of Company's recent SEC filings, we understand that you
have a significant ownership position in Company. The Purchaser is unwilling
to enter into the Purchase Agreement absent your assurance regarding certain
matters addressed in Section 9.9 of the Purchase Agreement. In order to
further induce the Purchaser to enter into the Purchase Agreement and to
consummate the transactions contemplated thereby, the Purchaser hereby
requests that you agree as follows:

     1. Upon the written request of the Purchaser, following the closing
contemplated under the Purchase Agreement, you will promptly enter into an
agreement with the Purchaser in accordance with the terms of Section 9.9 of the
Purchase Agreement. Such agreements will be suspended at such times, if any,
during the Two-Year Period that the Purchaser's owned voting power in Company
falls below (a) 12% due to transfers of Company equity by the Purchaser and/or
(b) 10% for any other reason. Such agreement shall only be effective during the
Two-Year Period.

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BCC Acquisition II LLC
Page 2
June 6, 1999


     2. You believe that, as a shareholder of Company, you will derive
substantial indirect benefit from the Purchaser's execution, delivery and
performance of the Purchase Agreement and the consummation of the transactions
contemplated thereby.

     Please execute a copy of this letter in the space indicated to confirm your
agreement with the matters set forth in clauses (1) and (2) above, and forward
the signed letter by facsimile to the attention of Ross Katchman at
650/857-4837, with hard copy to follow by U.S. Mail.

     Thank you for your cooperation in this matter.


                                                 Sincerely,

                                                 HEWLETT-PACKARD COMPANY


                                                 By:  /s/ ROSS N. KATCHMAN
                                                      Name:  Ross N. Katchman
                                                      Title: Corporate Counsel

Agreed to and Accepted as for the date first above written:

BCC Acquisition II LLC


By:  /s/ FRED B CRAVES
     Name:  Fred B. Craves
     Title: Authorized Signatory
