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                                                                   EXHIBIT 10.15

                                PROMISSORY NOTE

    For value received, Michael N. McDonald, an individual, ("Maker"), promises
to pay to the order of UroCor, Inc., a Delaware corporation, on or before
December 11, 1998, the principal sum of Twenty Thousand Dollars ($20,000.00),
together with interest thereon from the date hereof at the rates hereinafter
specified, payable as follows:

    The unpaid principal amount from time to time outstanding under this Note
    shall bear interest at the rate of seven and one-half percent (7.5%) per
    annum. Maker has the right to draw an amount up to $20,000.00 from time to
    time under this Note and shall repay the loan in full on or before December
    11, 1998. Interest shall be computed on the basis of a 365 or 366 day year,
    as applicable.

    Payments of both principal and interest are to be made in the lawful money
    of the United States of America.

    Any sum not paid when due will bear interest at the rate per annum equal to
the Note rate plus five percent (5%), and such interest which has accrued will
be paid at the time of, and as a condition precedent to, the curing of any
default hereunder.

    At the option of the holder, the unpaid balance of this Note, and all other
obligations of Maker to the holder, whether direct or indirect, absolute or
contingent, now existing or hereafter arising, shall become immediately due and
payable without notice or demand upon the occurrence or existence of any of the
following events or conditions: (a) any payment required by this Note or any
other note or obligation of Maker to the holder or to others is not made when
due; (b) the Maker's termination of employment with UroCor, Inc. or its
successors; or (c) Maker becomes insolvent or generally fails to pay, or admits
in writing its inability to pay, debts as they become due; or the Maker applies
for, consents to, or acquiesces in the appointment of a trustee, receiver or
other custodian for the Maker, or makes a general assignment for the benefit of
creditors; or in the absence of such application, consent or acquiescence, a
trustee, receiver or other custodian is appointed for the Maker; or any
bankruptcy, reorganization, debt arrangement or other case or proceeding under
any bankruptcy or insolvency law is commenced in respect to the Maker.

    The Maker shall have the right to prepay this Note in whole or in part at
any time and from time to time without premium or penalty, but with interest to
the date of payment on the amount prepaid. All payments under this Note shall be
applied first to any accrued interest and then to principal.

    The Maker agrees that if, and as often as, this Note is placed in the hands
of an attorney for collection or to defend or enforce any of the holder's rights
hereunder, the Maker shall pay to the holder hereof its reasonable attorney's
fees, together with all court costs and other expenses incurred and paid by such
holder.

    The Maker, endorsers, sureties, guarantors and all other persons who may
become liable for all or any part of this obligation severally waive presentment
for payment, protest and notice of nonpayment.

    The failure of the holder hereof to exercise any of the remedies or options
set forth in this Note upon the occurrence of one or more events of default,
shall not constitute a waiver of the right to exercise the same or any other
remedy at any subsequent time in respect to the same or any other event of
default. The acceptance of the holder hereof of any payment which is less than
the total of all amounts due and payable at the time of such payment shall not
constitute a waiver of the right to exercise any of the foregoing remedies or
options at that time or any subsequent time, or nullify any prior exercise of
any such remedy or option, without the express consent of the holder hereof,
except as and to the extent otherwise provided by law.

    The records of the holder of this Note shall be prima facie evidence of the
amount owing on this Note. This Note may be assigned by holder without the prior
consent of Maker.

    This Note is made under and governed by the laws of the State of Oklahoma.
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    IN WITNESS WHEREOF, the undersigned Maker has executed this instrument
effective the 11th day of December, 1997.

<TABLE>
<S>                             <C>  <C>
                                "Maker":

                                              /s/ MICHAEL N. MCDONALD
                                     -----------------------------------------
                                                Michael N. McDonald
</TABLE>