Exhibit 10.21

                    Amendment No. 1 to Stock Option Agreement

      This Amendment No. 1, dated as of September 17, 1997, to the Stock Option
Agreement dated as of February 25, 1994 with respect to 50,000 option shares at
an exercise price of five and five-eighths dollars ($5.625) per share (the
"Stock Option Agreement"), is entered into by and between Unilab Corporation, a
Delaware corporation (the "Corporation") and Richard A. Michaelson (the "Option
Holder").

      The Stock Option Agreement is hereby amended as follows:

      1. Section 1 is amended by deleting the final sentence thereof, with the
         effect of immediately vesting all Options.

      2. Section 3 is amended by (i) adding to the first sentence thereof, after
         the word "to" and before the word "a", the words "all or"; and (ii)
         deleting the following words from the beginning of the third sentence
         of the section: "Unless terminated earlier pursuant to Paragraph 6
         hereof,", so that the sentence reads in its entirety as follows: "Any
         Option not exercised within such time shall terminate."

      3. Sections 6(a) and 6(b) are deleted in their entirety and replaced by
         the following: "Option Holder or his estate shall be entitled to
         exercise the Option through the Expiration Date notwithstanding
         termination of Option Holder's employment by the Corporation or Option
         Holder's death prior to the Expiration Date".

      4. Section 9 is hereby deleted in its entirety and replaced with the
         following:

            9. Mergers, Recapitalizations and Dissolutions. The "acquisition" of
               the Corporation by another entity shall cause each outstanding
               option to become an option to purchase shares of the acquiring
               entity, for the balance of the term of the option, at a price and
               for a number of shares as is consistent with the acquisition
               terms, notwithstanding any contrary provision in the Stock Option
               Program in accordance with which this option was granted. The
               "acquisition" of the Corporation by another entity shall be
               defined to be either a merger or consolidation with an acquiring
               entity (or subsidiary or affiliate thereof) in which the
               Corporation is not the surviving entity or in which the
               Corporation becomes a subsidiary of an acquiring entity; the sale
               of substantially all of the Corporation's assets; or the
               dissolution or liquidation of the Corporation.
<PAGE>

      Except as expressly provided above, the Stock Option Agreement shall
remain in full force and effect.

      This Amendment No. 1 shall be governed by and construed in accordance with
the laws of the State of Delaware, other than its conflicts of laws provisions.

      OPTION HOLDER                             UNILAB CORPORATION

                                          By:
      --------------------------              ----------------------------------
Name: Richard A. Michaelson               Name:
                                          Title: