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                                                                   EXHIBIT 10.20




                                AMENDMENT NO. 1
               (DATED APRIL 21, 1998 FOR REFERENCE PURPOSES ONLY)
                                        
                          TO LEASE DATED MAY 10, 1996
                                        
               BETWEEN PRESBYTERIAN HEALTH FOUNDATION AS LANDLORD
                          AND ZYMETX, INC., AS TENANT


WHEREAS, Landlord and Tenant are the parties to the above described Lease for
the Building located at 800 N. Research Parkway; and,

Landlord and Tenant mutually desire to amend the above described Lease, (amended
via Addendum No. 1) as follows:

The provisions of this Amendment No. 1 shall supercede and control any
inconsistent provisions contained in the Lease, regardless of whether such
inconsistent provisions are contained in the printed portion of the lease or any
rider, addendum or exhibit annexed thereto.


1.  Building:                      800 Research Parkway Building I, Oklahoma 
                                   City, OK

2.  Amended Leased Premises:       The Leased Premises will change to include
                                   all of the 1st floor consisting of
                                   approximately 22,833 rentable square feet
                                   (RSF) and all of the 2nd floor consisting of
                                   22,800 RSF. Tenant currently occupies 11,803
                                   RSF. This equates to an overall increase of
                                   approximately 33,830 RSF for a total of
                                   45,633 RSF.

3.  Effective Commencement:        December 1, 1998 for the 2nd floor space
                                   (22,800 RSF) and March 1, for the remaining
                                   10,997 RSF of the 1st floor. Tenant shall
                                   occupy space within the Premises upon the
                                   substantial completion of the Tenant
                                   Improvement's of said space.

4.  Amended Tenant Improvement:    Landlord will contribute a Tenant Improvement
                                   Allowance for any new Allowance: space leased
                                   greater than the existing space of 11,803
                                   RSF, such allowance shall be the lesser of:

                                   o ($0.166 x months remaining on lease)
                                     x (rentable square feet leased greater
                                     than 11,803 RSF)

                                                       OR

                                   o The actual cost of the Tenant Improvements.

                                   Any excess cost of the Tenant Improvements
                                   shall be the responsibility of the Tenant.

5.  Lease Rate:                    The lease rate for any space greater than
                                   11,803 RSF shall be $12.00 per RSF annually.

6.  Amended Security Deposit:      Tenant shall pay an amount that is equal to
                                   one month's rent based on the additional
                                   square footage.

7.  Option to Expand:              Landlord hereby grants Tenant a right of
                                   first refusal to expand into the 3rd floor
                                   ("Additional Space") of the Building as
                                   provided in this paragraph. Landlord will
                                   provide Tenant written notice of Landlord's
                                   intent to lease any such space to a third
                                   party.  Tenant may exercise such right within
                                   fifteen (15) days of receiving such notice by
                                   executing a written amendment to this Lease
                                   covering the Additional Space, prior to the
                                   time Landlord commits to lease such space to
                                   a third party.  Such amendment shall provide
                                   for the leasing of such space on the same
                                   terms and conditions contained in this
                                   Amendment No. 1.  In such event (a) the
                                   Additional Space shall be included as part of
                                   the Leased Premises, (b) the lease for the
                                   Additional Space shall be co-terminous with
                                   the lease for the Leased Premises, (c) the
                                   lease rate shall be $12.00 per RSF annually
                                   for an such Additional Space and (d) the
                                   Tenant Improvements Allowance shall be based
                                   on the lesser of: (i) $0.166 x months
                                   remaining on lease) x (rentable square feet
                                   of Additional Space) or (ii) the actual cost
                                   of such Tenant Improvements for the
                                   Additional Space.



Both parties agree to the terms of this Lease Agreement as amended by Addendum
No. 1 and this Amendment No. 1 and the lease Agreement shall continue in full
force.

Landlord:  Presbyterian Health Foundation           Tenant: ZYMETX, INC.


                                               /s/ PETER G. LIVINGSTON
____________________________________         ___________________________________
Dennis McGrath, Vice President                 Peter G. Livingston, President