<PAGE>
                                                                EXHIBIT 10.18(a)

THE SECURITIES REPRESENTED BY THIS NOTE AND ISSUABLE UPON EXERCISE HEREOF HAVE
NOT BEEN REGISTERED UNDER THE UNITED STATES SECURITIES ACT OF 1933, AS AMENDED
(THE "1933 ACT"), OR UNDER THE PROVISIONS OF ANY APPLICABLE STATE SECURITIES
LAWS, BUT HAVE BEEN ACQUIRED BY THE REGISTERED HOLDER HEREOF FOR PURPOSES OF
INVESTMENT AND IN RELIANCE ON STATUTORY EXEMPTIONS UNDER THE 1933 ACT, AND UNDER
ANY APPLICABLE STATE SECURITIES LAWS. THESE SECURITIES AND THE SECURITIES ISSUED
UPON EXERCISE HEREOF MAY NOT BE SOLD, PLEDGED, TRANSFERRED OR ASSIGNED, NOR MAY
THIS NOTE BE EXERCISED, EXCEPT IN A TRANSACTION WHICH IS EXEMPT UNDER THE
PROVISIONS OF THE 1933 ACT AND ANY APPLICABLE STATE SECURITIES LAWS OR PURSUANT
TO AN EFFECTIVE REGISTRATION STATEMENT.



                                 ASPENBIO, INC.

                         6% CONVERTIBLE PROMISSORY NOTE


$500,000 U.S.                                                       JULY 5, 2002

         FOR VALUE RECEIVED, ASPENBIO, INC., a Colorado corporation (the
"COMPANY"), hereby promises to pay to the order of MICHAEL S. SMITH, an
individual (the "HOLDER"), in lawful money of the United States at the address
of Holder set forth below, the principal amount of $500,000, together with
Interest (as hereinafter defined), which such Interest shall accrue from the
date hereof until the date of payment in full of the aggregate principal amount
of this Note or the conversion of this Note pursuant to the terms hereof.

         This Note is intended to provide Company (a) cash for a liquid asset
accounting containing $350,000 (the "ACCOUNT") at FirstBank of Tech Center (the
"BANK"), and (b) $150,000 for general operating expenses. The Account shall
serve as collateral (the "COLLATERAL") for Company's construction loan, which it
is obtaining from the Bank (the "CONSTRUCTION LOAN") for the construction of a
building located at the Southwest Terminus of South Perry Street, Castle Rock,
Colorado (the "BUILDING").

         This Note has been executed by Company on the date set forth above (the
"EFFECTIVE DATE").

         1. Interest. Except as otherwise provided herein, interest shall accrue
on the outstanding principal amount of this Note at the rate of 6% per annum,
calculated on the basis of the number of days elapsed in a 360-day year. Upon
the occurrence of an Event of Default and for so long as such Event of Default
continues, Interest shall accrue on the outstanding principal amount of this
Note at the rate per annum of the lower of 18% or the maximum rate of interest
permissible under any applicable law at any time (the "DEFAULT INTEREST RATE").
The term "INTEREST" shall include both the 6% per annum and the Default Interest
Rate.

         2. Maturity. Unless this Note is earlier converted into shares of the
Company's Common Stock (as defined herein), the principal of and accrued
Interest on this

<PAGE>

Note is due and payable in a single lump sum payment on the earliest to occur of
(collectively, the "MATURITY DATE"):

            2.1. the 12 month anniversary of the Effective Date; or

            2.2. (i) upon the Bank's release of the entire Account and
Collateral or (ii) upon the closing of a permanent loan to take out the
Construction Loan, provided that in no event shall the Maturity Date occur prior
March 31, 2003, unless Holder consents in writing (which consent may be withheld
in Holder's sole discretion);

provided, however, that upon the occurrence of an Event of Default (as
hereinafter defined), all unpaid principal and accrued Interest on this Note
shall immediately become due and, in the case of an Event of Default described
in Sections 11.1 or 11.2 payable upon the written demand of Holder and, in the
case of an Event of Default described in Section 11.3, without any action by
Holder. Upon payment in full of all principal and Interest payable hereunder,
this Note shall be surrendered to Company for cancellation.

         3. Application of Payments.

            3.1. Except as otherwise expressly provided herein, each payment of
outstanding principal amount and Interest on this Note shall be applied (i)
first to the repayment of any sums incurred by Holder for the payment of any
expenses in enforcing the terms of this Note, (ii) then to the payment of
Default Interest, (iii) then to the payment of Interest, and (iv) then to the
reduction of the principal.

            3.2. Upon payment in full of the principal of, and accrued and
unpaid Interest on, this Note, this Note shall be marked "Paid in Full" and
returned to Company.

         4. Prepayment. This Note may be prepaid in part or in full at any time
after March 31, 2003, but not before such date.

         5. Note Conversion.

            5.1. Conversion. At any time on or prior to the Maturity Date,
Holder may elect to convert all, or any part, of the outstanding principal
balance of this Note and all Interest accrued and unpaid thereon into shares of
common stock of the Company (the "COMMON STOCK") at a conversion price per share
of Common Stock of $1.50, as adjusted and readjusted from time to time in
accordance with Section 5.3 (such conversion price, as so adjusted and
readjusted and in effect at any time, being herein call the "CONVERSION PRICE").

            5.2. Conversion Procedure.

                 5.2.1 Notice of Conversion. Holder shall delivered to Company
written notice that Holder is exercising his conversion right pursuant to
Section 5.1 ("CONVERSION NOTICE"). Such Conversion Notice shall set forth (i)
the principal amount of this Note and the amount of accrued Interest that Holder
intends to convert and (ii) the date on which such conversion will occur. All
amounts converted shall be applied first to any accrued, but unpaid Interest,
then to the reduction of principal.


                                       2
<PAGE>

                 5.2.2 Delivery of Stock Certificates. As promptly as
practicable after the conversion of this Note, Company at its expense will issue
and deliver to Holder a certificate(s) for the number of full shares of Common
Stock issuable upon such conversion.

                 5.2.3 Delivery of Replacement Note. Upon the conversion of this
Note, Holder shall surrender this Note, duly endorsed, at the principal office
of Company. If Holder only converts part of the principal amount of this Note,
as promptly as practicable after the conversion of that portion of this Note,
Company at its expense will issue and deliver to Holder a new principal amount
of this Note, Company shall be forever released from all its obligations and
liabilities under this Note.

                 5.2.4 Fractional Shares. No fractional shares of the Company's
Common Stock shall be issued upon conversion of this Note. In lieu of Company
issuing any fractional shares to Holder upon the conversion of this Note, the
number of shares of Common Stock to be issued shall be rounded up to the next
whole number of shares.

            5.3. Adjustment of Conversion Price. The Conversion Price shall be
subject to adjustment from time to time as follows:

                 5.3.1 If Company shall issue any Additional Stock, or rights to
acquire Additional Stock (as hereinafter defined) for a consideration per share
less than the Conversion Price immediately prior to the issuance of such
Additional Stock (the "LOWER PRICE"), the Conversion Price for this Note in
effect immediately prior to each such issuance shall be reduced to the Lower
Price.

                 5.3.2 In the case of the issuance of Common Stock for cash, the
consideration shall be deemed to be the amount of cash paid therefor before
deducting any reasonable discounts, commissions or other expenses allowed, paid
or incurred by Company for any underwriting or otherwise in connection with the
issuance and sale thereof.

                 5.3.3 In the case of the issuance of Common Stock for a
consideration in whole or in part other than cash, the consideration other than
cash shall be deemed to be the fair value thereof as determined in good faith by
the Company's board of directors (the "BOARD OF DIRECTORS").

                 5.3.4 "ADDITIONAL STOCK" shall mean any shares of Common Stock
issued by Company after the Effective Date hereof except:

                       5.3.4.1 Common Stock issued pursuant to a transaction
described in Section 5.3.5;

                       5.3.4.2 400,000 shares of Common Stock issuable to
employees, directors, officers or consultants of Company pursuant to stock
options outstanding on the Effective Date under the 2002 Stock Incentive Plan;

                       5.3.4.3 200,000 shares of Common Stock issuable to
directors of the Company pursuant to stock options outstanding on the Effective
Date;

                       5.3.4.4 830,000 shares of Common Stock issuable pursuant
to warrants outstanding on the Effective Date; or


                                       3
<PAGE>

                       5.3.4.5 up to 5% of the issued and outstanding Common
Stock as of the Effective Date, issued upon approval of the Company's Board of
Directors in connection with any agreement between Company and a third party
regarding such third party's development for or together with or sale to Company
of technology, know-how or intellectual property.

                 5.3.5 In the event Company should at any time or from time to
time after the Effective Date fix a record date for the effectuation of a split
or subdivision of the outstanding shares of Common Stock, declares a dividend or
other distribution payable in additional shares of Common Stock or other
securities or rights convertible into, or entitling the holder thereof to
receive, directly or indirectly, additional shares of Common Stock (hereinafter
referred to as "COMMON STOCK EQUIVALENTS"), combine its outstanding shares of
Common Stock into a lesser number of shares or issue by reclassification of its
shares of Common Stock any shares of its capital stock, without payment of any
consideration by such holder for the additional shares of Common Stock or the
Common Stock Equivalents (including the additional shares of Common Stock
issuable upon conversion or exercise thereof), then, as of such record date (or
the date of such dividend, distribution, split, subdivision, combination or
reclassification if no record date is fixed), the Conversion Price shall be
appropriately adjusted so that the number of shares of Common Stock issuable on
conversion of this Note shall be adjusted in proportion to such change in the
number of outstanding shares.

         6. Unsecured and Subordinated. The indebtedness represented by this
Note is secured pursuant to that Pledge Agreement, dated as of the date hereof,
executed by Company in favor of Holder (the "PLEDGE AGREEMENT").

         7. Waiver of Notice. The Company hereby waives diligence, notice,
presentment, protest and notice of dishonor.

         8. Transfer of this Note or Common Stock on Conversion Hereof. This
Note and the securities into which this Note may be converted may be
transferred, provided that such transfer complies with any applicable securities
laws.

         9. Representations and Warranties of Company.

            9.1. Due Incorporation and Good Standing. Company is a corporation
duly organized, validly existing and in good standing under the laws of the
State of Colorado, with full and adequate power to carry on and conduct its
business as presently conducted, and is duly licensed or qualified in all
foreign jurisdictions wherein the failure to be so qualified or licensed would
reasonably be expected to have a material adverse effect on the business of
Company.

            9.2. Due Authorization. Company has full right, power and authority
to enter into this Note, to make the borrowings and execute and deliver this
Note as provided herein and to perform all of its duties and obligations under
this Note. The execution and delivery of this Note will not, nor will the
observance or performance of any of the matters and things herein or therein set
forth, violate or contravene any provision of law or Company's bylaws or
articles of incorporation. All necessary and appropriate corporate action on the
part of Company has been taken to authorize the execution and delivery of this
Note.


                                       4
<PAGE>

            9.3. Enforceability. This Note has been validly executed and
delivered by Company and constitutes the legal, valid and binding obligations of
Company enforceable against it in accordance with its respective terms, subject
to applicable bankruptcy, insolvency, reorganization or similar laws relating to
or affecting the enforcement of creditors' right and to the availability of the
remedy of specific performance.

            9.4. Capitalization. All of Company's authorized and outstanding
equity securities (including securities convertible into equity securities) are
identified in the Company's Form S-1, Registration No. 333-86190, as filed with
the Securities and Exchange Commission on June 6, 2002.

            9.5. Compliance with Laws. The nature and transaction of Company's
business and operations and the use of its properties and assets do not and
during the term of this Note shall not, violate or conflict with in any material
respect any applicable law, statute, ordinance, rule, regulation or order of any
kind or nature.

            9.6. Absence of Conflicts. The execution, delivery and performance
by Company of this Note, and the transactions contemplated hereby, do not
constitute a breach or default, or require consents under, any agreement,
permit, contract or other instrument to which Company is a party, or by which
Company is bound or to which any of the assets of Company is subject, or any
judgment, order, writ, decree, authorization or license to which any Company,
the assets of Company is bound or subject or any rule, regulations or statutes
and will not result in the creation of any lien upon any of the assets of
Company.

            9.7. Issuance Upon Conversion. Company shall reserve an adequate
number of shares of Common Stock for conversion of this Note. Upon conversion of
this Note, the Common Stock shall be validly issued, fully paid and
nonassessable, and Company shall use its reasonable efforts to ensure that the
Common Stock issuable upon conversion of this Note will be issued in accordance
with exemptions under applicable federal and state securities laws.

            9.8. Litigation and Taxes. There is no litigation or governmental
proceeding pending, or to the best knowledge of Company after due inquiry,
threatened, against Company. Company has duly filed all applicable income or
other tax returns and has paid all material income or other taxes when due.
There is no controversy or objection pending, or to the best knowledge of
Company after due inquiry, threatened in respect of any tax returns of Company.

            9.9. Indebtedness; Liens; Material Contracts. Except as set forth in
Schedule 1 attached hereto, (a) Company has incurred no indebtedness, or liens
or encumbrances on any of its assets, other than pursuant to purchase-money
loans or leases in the ordinary course of business, in all cases not exceeding
$50,000 in the aggregate and (b) Company has not entered into any contracts
involving payments of more than $50,000 in the aggregate or that are otherwise
material to Company's business.

            9.10. No Omissions or Misstatements. None of the information
included in this Note or other documents or information furnished or to be
furnished by Company, or any of its representations, contains any untrue
statement of a material fact or is misleading in any


                                       5
<PAGE>
material respect or omits to state any material fact. Copies of all documents
referred to in herein have been delivered or made available to Holder and
constitute true and complete copies thereof and include all amendments,
schedules, appendices, supplements or modifications thereto or waivers
thereunder.

         10. Covenant of Company. If payment for a Cost Overrun (as defined
below) will be satisfied by withdrawal or use of funds from the Account, Company
shall obtain the prior written approval from Holder before allowing or approving
any change orders that would increase the cost of constructing the Building over
the estimated cost of $3,627,199 (a "COST OVERRUN"). Holder agrees to use
commercially reasonable judgment with respect to his decision to approve such
Cost Overruns; provided, however, that if at anytime the Account has less than
$200,000, then Holder's decision to approve such Cost Overruns shall be in his
sole and absolute discretion.

         11. Events of Default. The occurrence of any of following events (each
an "EVENT OF DEFAULT"), not cured in the applicable cure period or grace period,
if any, shall constitute an Event of Default of Company:

             11.1. a material breach of any representation, warranty, covenant
or the other provisions of this Note, which is not cured within 5 days following
notice thereof to Company;

             11.2. the failure to make when due any payment described in this
Note;

             11.3. (i) the application for the appointment of a receiver or
custodian for Company or the property of Company, (ii) the entry of an order for
relief or the filing of a petition by or against Company under the provisions of
any bankruptcy or insolvency law, (iii) any assignment for the benefit of
creditors by or against Company, (iv) Company becomes insolvent; or (v)
Company's default under the Construction Loan.

         12. Miscellaneous.

             12.1. Successors and Assigns. Subject to the exceptions
specifically set forth in this Note, the terms and conditions of this Note shall
inure to the benefit of and be binding upon the respective executors,
administrators, heirs, successors and assigns of the parties.

             12.2. Loss or Mutilation of Note. Upon receipt by Company of
evidence satisfactory to Company of the loss, theft, destruction or mutilation
of this Note, together with indemnity reasonably satisfactory to Company, in the
case of loss, theft or destruction, or the surrender and cancellation of this
Note, in the case of mutilation, Company shall execute and deliver to Holder a
new Note of like tenor and denomination as this Note. Principal is payable only
to the registered Holder of this Note.

             12.3. Titles and Subtitles. The titles and subtitles of the
Sections of this Note are used for convenience only and shall not be considered
in construing or interpreting this agreement.


                                       6
<PAGE>

             12.4. Legend. Any certificate representing shares of Company's
Common Stock issued upon conversion of this Note or otherwise issued hereunder
shall be stamped or otherwise imprinted with a legend substantially in the
following form:

THESE SECURITIES HAVE NOT BEEN REGISTERED UNDER THE UNITED STATES SECURITIES ACT
OF 1933, AS AMENDED (THE "1933 ACT"), OR UNDER THE PROVISIONS OF ANY APPLICABLE
STATE SECURITIES LAWS, BUT HAVE BEEN ACQUIRED BY THE REGISTERED HOLDER HEREOF
FOR PURPOSES OF INVESTMENT AND IN RELIANCE ON STATUTORY EXEMPTIONS UNDER THE
1933 ACT, AND UNDER ANY APPLICABLE STATE SECURITIES LAWS. THESE SECURITIES AND
THE SECURITIES ISSUED UPON EXERCISE HEREOF MAY NOT BE SOLD, PLEDGED, TRANSFERRED
OR ASSIGNED, NOR MAY THIS NOTE BE EXERCISED, EXCEPT IN A TRANSACTION WHICH IS
EXEMPT UNDER THE PROVISIONS OF THE 1933 ACT AND ANY APPLICABLE STATE SECURITIES
LAWS OR PURSUANT TO AN EFFECTIVE REGISTRATION STATEMENT.

             12.5. Notices. Any notice, request or other communication required
or permitted hereunder shall be in writing and shall be delivered personally or
by facsimile (receipt confirmed electronically) or shall be sent by a reputable
express delivery service or by certified mail, postage prepaid with return
receipt requested, addressed as follows:

                  if to Company to:

                  AspenBio, Inc.
                  8100 Southpark Way, Building B-1
                  Littleton, Colorado 80120
                  Attn:    Roger D. Hurst
                  Fax:     (303) 794-2000

                  with a copy to:

                  Krendl Krendl Sachnoff & Way PC
                  370 17th Street, Suite 5350
                  Denver, Colorado  80202
                  Telephone:  (303) 629-2600
                  Facsimile :  (303) 629-2606
                  Attention:  Cathy S. Krendl, Esq.

                  and

                  Patton Boggs LLP
                  1660 Lincoln Street, Suite 1900
                  Denver, Colorado  80202
                  Telephone:  (303) 830-1776
                  Facsimile:  (303) 894-9239
                  Attention:  Robert M. Bearman, Esq.


                                       7
<PAGE>

                  if to Holder to:

                  Michael S. Smith
                  c/o The Kaitar Foundation
                  1660 Lincoln St., Suite 1420
                  Denver, CO  80264
                  Fax:     (303) 832-9015

                   with a copy to:

                  Brownstein Hyatt & Farber, P.C.
                  410 Seventeenth Street, 22nd Floor
                  Denver, CO  80202
                  Attn:  Steven Demby. Esq.
                  Fax No.:  (303) 223-0919

Either party hereto may change the above specified recipient or mailing address
by notice to the other party given in the manner herein prescribed. All notices
shall be deemed given on the day when actually delivered as provided above (if
delivered personally or by facsimile, provided that any such facsimile is
received during regular business hours at the recipient's location) or on the
day shown on the return receipt (if delivered by mail or delivery service).

             12.6 Note Holder Not Shareholder. This Note does not confer upon
Holder any right to vote or to consent to or to receive notice as a shareholder
of the Company, as such, in respect of any matters whatsoever, or any other
rights or liabilities as a shareholder, prior to the conversion hereof.

             12.7 Governing Law. The terms of this Note shall be construed in
accordance with the laws of the State of Colorado. The jurisdiction and venue
shall be in court situated in the City and County of Denver, Colorado.

             12.8 Waiver and Amendment. Any term of this Note may be amended,
waived or modified with the written consent of Company and Holder of this Note.

             12.9. Remedies; Attorneys Fees. No delay or omission by Holder in
exercising any of its rights, remedies, powers or privileges hereunder or at law
or in equity and no course of dealing between Holder and the undersigned or any
other person shall be deemed a waiver by Holder of any such rights, remedies,
powers or privileges, even if such delay or omission is continuous or repeated,
nor shall any single or partial exercise of any right, remedy, power or
privilege preclude any other or further exercise thereof by Holder or the
exercise of any other right, remedy, power or privilege by Holder. The rights
and remedies of Holder described herein shall be cumulative and not restrictive
of any other rights or remedies available under any other instrument, at law or
in equity. If an Event of Default occurs, Company agrees to pay, in addition to
the principal and Interest payable hereunder, reasonable attorneys' fees and any
other costs incurred by Holder in connection with its pursuit of its remedies
under this Note.



                                    * * * * *


                                       8
<PAGE>

         IN WITNESS WHEREOF, Company has caused this Note to be signed in its
name this 5th day of July, 2002.


ASPENBIO, INC.

By:
         -------------------------------
           Name:
                  ----------------------
           Title:
                  ----------------------

<PAGE>

                                   SCHEDULE 1


1.   Promissory Note issued to Roger Hurst, approximate outstanding principal
     amount of $625,000 as of the Effective Date.

2.   Promissory Note issued to Roger Hurst, approximate outstanding principal
     amount of $267,500 as of the Effective Date.

3.   Promissory Note issued to Roger Hurst, approximate outstanding principal
     amount of $29,775 as of the Effective Date.

4.   Equipment Lease with Colorado Business Leasing, approximate outstanding
     principal amount of $150,000 as of the Effective Date.

5.   License Agreement with the University of Wyoming.

6.   License Agreement with the University of Idaho.


