Exhibit 10.15

                             SECURED PROMISSORY NOTE


$250,000.00                                                  Tarzana, California
                                                             January 20, 1997


         The undersigned, David C. Weavil, a resident of the State of North
Carolina (the "Borrower"), hereby promises to pay to Unilab Corporation, a
Delaware corporation ("Unilab"), the principal sum of $250,000.00, payable in
lawful money of the United States and in immediately available funds, plus
interest at the rate of 6% per annum, payable quarterly in arrears.

         The principal sum shall be due on January 20, 2002 ("Maturity Date")
and on such date the Borrower shall (or if such day is a day on which banks in
Los Angeles are not open for business, on the immediately following banking
day), pay to Unilab in immediately available funds, the full principal sum of
$250,000.00 plus accrued and unpaid interest thereon.

         This Note and the obligation created hereunder is secured and
collateralized by the shares of Unilab common stock ("Shares") purchased by the
Borrower with the proceeds hereof pursuant to that certain Employment Agreement,
dated January 20, 1997, between Unilab and the Borrower (the "Employment
Agreement"). If the principal sum of $250,000.00, plus accrued and unpaid
interest thereon, is not paid by the Borrower to Unilab on or before the
Maturity Date as set forth in the immediately preceding paragraph, Unilab shall
have the right to take, liquidate withhold or cancel such number of Shares sold
to the Borrower under the Employment Agreement as shall have a value (based upon
the closing market price of Unilab common stock on the Maturity Date) equal to
the amount remaining unpaid under this Note at such date. Except as set forth in
this paragraph, Unilab shall have no recourse against the Borrower for the
Borrower's default hereunder.

         The Borrower represents and warrants as follows: (a) the Borrower is a
resident of the State of North Carolina and (b) this Note constitutes the legal,
valid and binding obligation of the Borrower, enforceable against the Borrower
in accordance with its terms.

         The Borrower may, at its option and upon written notice to Unilab,
prepay the outstanding principal amount of this Note in whole or in part,
without penalty or premium.

         Upon default in Borrower's payment of this Note, Unilab shall have the
right to exercise any and all of its rights under this Note.

<PAGE>

         All notices or other communications provided for hereunder shall be in
writing (including telecommunications) and shall be mailed, telecopied or
delivered to (a) the Borrower at the address of the Borrower set forth
underneath his signature, or at such other address as may hereafter be specified
by the Borrower to Unilab, or (b) Unilab, at its principal executive offices at
18448 Oxnard Street, Tarzana, California 91356, Attention: Corporate Secretary.
All notices and communications shall be effective (i) if mailed, when received
or three days after mailing, whichever is earlier, (ii) if telecopied, when
transmitted, and (iii) if delivered, upon delivery.

         No failure on the part of Unilab to exercise, and no delay in
exercising, any right, power, privilege or remedy hereunder shall operate as a
waiver thereof, nor shall any single or partial exercise thereof by Unilab
preclude any other or further exercise thereof or the exercise of any other
right, power, privilege or remedy of Unilab. No amendment or waiver of any
provision of this Note, nor consent to any departure by the Borrower therefrom,
shall in any event be effective unless the same shall be in writing and signed
by Unilab and then such waiver or consent shall be effective only in the
specific instance and for the specific purpose for which given. This Note shall
be governed by, and construed in accordance with, the laws of the State of
California, without regard to principles of conflicts of law.

         The Borrower hereby agrees to pay on demand all costs and expenses
(including, without limitation, all fees, expenses and other client charges of
counsel to Unilab) incurred by Unilab in connection with the enforcement of
Unilab's rights, and in the collection of all amounts due, hereunder.

                                       DAVID C. WEAVIL



                                       By: ___________________________________
                                           Address: