Compensation  charged by real estate brokers is not set by law. Such charges are
established by each real estate broker.

DIFFERENT  BROKERAGE  RELATIONSHIPS  ARE  AVAILABLE  WHICH INCLUDE BUYER AGENCY,
SELLER AGENCY, SUBAGENCY OR TRANSACTION-BROKER.

                    EXCLUSIVE RIGHT-TO-SELL LISTING CONTRACT
                          (FARM AND RANCH/VACANT LAND)
                                 (SELLER AGENCY)

                                                     Denver, Colorado   9/17/97


SOMATOGEN,  INC.  ("Seller") hereby  irrevocably  appoint(s) CB Commercial  Real
Estate  Group,  Inc.,  1050 - 17th Street,  Suite 800,  Denver,  Colorado  80265
("Broker")  as  Seller's  exclusive  agent for the  purposes and under the terms
specified herein, and the parties agree:

     1.  PURPOSE  OF  AGENCY.  The  purpose of this  agency  contract  ("Listing
Contract")  is to engage the  efforts of Broker to  accomplish  the sale of real
property legally described as:

                          Lot 8 Flatiron Industrial Park 5

also known as 2400 - 55th Street,  Boulder,  Colorado  80301, together  with the
following water rights:  None the following  water stock:  None all well rights,
reservoir or storage rights, if any; all pumping or well equipment, if any, the
following  growing crops: None and all items of personal property to be conveyed
pursuant to Section 8 (collectively, the "Property").

     2.  BROKER'S  SERVICES.  Broker is a limited  agent of the  Seller and will
represent only Seller.
         (a)  Broker  shall  promote the interests of the Seller with the utmost
              good faith, loyalty and fidelity,  including, but not limited to:
              (1) Seeking a price and terms which are  acceptable to the Seller,
              except that Broker shall not be obligated to seek additional  
              offers to purchase the Property while the Property is subject to a
              contract for sale;  

<PAGE> 2

              (2)  Presenting  all offers to and from the Seller in a timely  
              manner regardless of whether the Property is subject to a contract
              for sale;
              (3) Disclosing to the Seller adverse material facts actually known
              by the Broker;
              (4) Counseling the Seller as to any material  benefits or risks of
              a  transaction  actually  known by the Broker;  
              (5)  Advising  the Seller to obtain expert advice as to material  
              matters about which the Broker knows but the specifics of which 
              are beyond the expertise of the Broker;
              (6) Accounting in a timely manner for all money and property
              received; and
              (7) Informing  the  Seller  that such  Seller  may be  vicariously
              liable  for the acts of such  Seller's  agent or any  subagent
              when the  Broker is  acting  within  the  scope of the  agency
              relationship;
         (b)  Broker shall not disclose the  following  information  without the
              informed consent of the Seller:  
              (1) That the Seller is willing to accept less than the asking 
              price for the  Property;  
              (2) What the motivating  factors are for the Seller to sell the  
              Property;
              (3) That the Seller  will agree to  financing  terms  other than 
              those offered;  
              (4) Any material information about the Seller unless the
              disclosure is required by law or failure to disclose such
              information would constitute fraud or dishonest dealing; or
              (5) Any  facts or  suspicions  regarding  circumstances  which may
              psychologically   impact  or  stigmatize   any  real  property
              pursuant to Colorado law.
         (c)  Broker  shall  disclose  to  any  prospective  buyer  all  adverse
              material facts actually known by Broker  including but not limited
              to adverse  material facts pertaining to the title to the Property
              and the physical  condition of the Property,  any material defects
              in the  Property,  and any  environmental  hazards  affecting  the
              Property which are required by law to be disclosed.

     3. SALE.  "Sale of the Property" or "Sale" means the voluntary  transfer or
exchange of any interest in the Property or the voluntary  creation of the right
to acquire any interest in the Property.

     4. EFFECT OF THIS LISTING CONTRACT.  By this appointment,  Seller agrees to
conduct all negotiations  for the Sale of the Property only through Broker,  and
to refer to Broker all inquiries  received in any form from real estate brokers,
salespersons,  perspective  buyers,  tenants or any other source during the time
this Listing  Contract is in effect.  Seller  authorizes  Broker to disclose any
facts  about  the  Property.   In  addition,   Seller  agrees  that  any  Broker
compensation which is conditioned upon the Sale of the Property shall be earned
by Broker as set forth herein  without any discount or allowance for any efforts
made by Seller or by any representative of Seller in connection with the Sale of
the Property.

<PAGE> 3

     5. THE LISTING  PERIOD.  Broker's  authority shall begin September 17, 1997
and shall continue through September 17, 1998 ("Listing Period").

     6.  PRICE AND TERMS.  Price: U.S. $3,500,000.00    Terms:  Cash.  Minimum
amount of earnest money deposit - Acceptable to owner.

     7. DEPOSITS. Broker is authorized to accept earnest money deposits pursuant
to a proposed Sale  contract.  Broker is authorized to deliver the earnest money
deposit to the  closing  agent,  if any,  at or before  the  closing of the Sale
contract.

     8. Intentionally omitted from contract.

     9. TITLE AND  ENCUMBRANCES.  Seller  represents to Broker that title to the
Property is solely in Seller's name.  Seller shall deliver to Broker true copies
of all relevant title materials,  lease(s) and survey(s) in Seller's  possession
and shall disclose to Broker all easements,  liens and encumbrances,  if any, on
the Property of which Seller has knowledge.  Seller authorizes the holder of any
obligation  secured by an  encumbrance on the Property to disclose to Broker the
amount owing on said encumbrance and the terms thereof.

         In case of Sale,  Seller agrees to convey,  by a special Warranty deed,
only that title Seller has in the Property.  All monetary  encumbrances (such as
mortgages,  deeds of trust, liens, financing statements) shall be paid by Seller
and released except as Seller and buyer may otherwise agree.  Existing  monetary
encumbrances  are as follows:  none.  The  Property is subject to the  following
leases and tenancies: none.

     10. EVIDENCE OF TITLE. Seller agrees to furnish buyer, at Seller's expense,
a commitment for, and a policy of, title insurance to the Property  certified to
a current date in the amount specified by any Sale contract.

     11. PRORATIONS.  General taxes for the year of closing,  based on the taxes
for the calendar year  immediately  preceding  closing,  rents,  water and sewer
charges,  owner's association dues and interest on continuing loans(s),  if any,
shall be prorated to date of closing.

     12.  POSSESSION.  Possession of the Property shall be delivered to buyer as
follows: upon delivery of deed, subject to leases and tenancies as described in
Section 9.

     13.  MATERIAL  DEFECTS -- BROKER  DISCLOSURES -- INSPECTION.  Seller agrees
that any defects of a material nature (including, but not limited to, structural
defects;  soil  conditions;  violations  of  health,  zoning or  building  laws;
nonconforming  uses and  zoning  variances)  actually  known by Broker  must be
disclosed by Broker to any prospective  buyer.  Seller agrees that any buyer may
have the Property and inclusions inspected.

<PAGE> 4

     14. COMMISSION TO BROKER.
         (a)  Commission.  In consideration of the services to be performed by 
              Broker, Seller agrees to pay Broker as follows:
              (1) Sale Commission: 6% of the gross sales price in U.S. dollars.
         (b)  When Earned. Such commission shall be earned upon the happening of
              any of the  following:  
              (1) Any Sale of the  Property  within  the Listing Period by
                  Seller, by Broker or by any other person;
              (2) Intentionally omitted from contract.
              (3) Any Sale of the Property within 90 calendar days subsequent to
                  the  expiration of the Listing Period  ("Holdover  Period") to
                  anyone  with  whom  Broker   negotiated  and  whose  name  was
                  submitted,  in writing, to Seller by Broker during the Listing
                  Period (including any extensions thereof).
         (c)  When Applicable and Payable.  The commission obligation shall 
              apply to a Sale made during the  Listing  Period or made during 
              any  extension  of such original or extended term. The commission 
              described in subsection (a)(1) shall be payable at the time of the
              closing of the Sale as contemplated by subsection (b)(1) or
              (b)(3).

     15.   LIMITATION  ON  BROKER'S   COMPENSATION.   Broker  shall  not  accept
compensation  from the buyer, the buyer's agent, or any entity  participating in
or providing services for the Sale without the written consent of Seller.

     16. OTHER BROKERS, ASSISTANCE -- MULTIPLE LISTING SERVICE.
         (a) Broker shall seek  assistance  from and offer  compensation  to the
             following brokers outside of the listing company:
              (1) Intentionally omitted from contract.
              (2) Brokers representing the buyer ("Buyer Agents"), (Buyer Agents
                  representing  the buyer  owe  duties  of  utmost  good  faith,
                  loyalty and fidelity to buyer only.  Seller is not vicariously
                  liable  for the  acts of  Buyer  Agents).  Broker  will  offer
                  compensation  to  Buyer  Agents  as  follows:   50%  of  total
                  commission.
              (3) Brokers assisting the buyer in the transactions but not acting
                  as agents ("Transaction-Brokers").  (Transaction-Brokers  must
                  exercise reasonable skill and care for the parties.  Seller is
                  not vicariously  liable for the acts of  Transaction-Broker).
                  Broker  will  offer  compensation  to   Transaction-Broker  as
                  follows:  50% of total commission.  
          (b)  Broker will not submit the Property to a multiple listing 
               service.

<PAGE> 5

     17. IN-COMPANY DUAL AGENCY AND TRANSACTION-BROKER. If a written Dual Agency
Addendum or  Transaction-Broker  Addendum  is signed by Seller,  Broker may show
Property to buyers represented or assisted by Broker.

     18.  FORFEITURE OF PAYMENTS.  In the event of a forfeiture of payments made
by a buyer,  the sums  received  shall be divided  between  Broker  and  Seller,
one-half thereof to Broker, but not to exceed the commission agreed upon herein,
and the balance to Seller.

     19. COST OF SERVICES; REIMBURSEMENT.  Unless otherwise specified in Section
28,  Broker  shall  bear all  expenses  incurred  by Broker,  if any,  to market
Property and to compensate  cooperating  brokers, if any. Broker will not obtain
or order any other  products or services  unless Seller agrees in writing to pay
for them  promptly  when  due.  Unless  otherwise  agreed,  Broker  shall not be
obligated  to advance  funds for the  benefit  of Seller in order to  complete a
closing (Examples:  surveys, radon tests, soil tests, title reports, engineering
studies).  Seller shall  reimburse  Broker for payments  made by Broker for such
other products or services authorized by Seller.

     20.  MAINTENANCE  OF THE  PROPERTY.  Seller agrees that Broker shall not be
responsible  for  maintenance  of the  Property  nor shall  Broker be liable for
damage of any kind occurring to the Property, unless such damage shall be caused
by the negligence of Broker.

     21. OTHER SELLERS. Seller acknowledges that Broker may have agreements with
other sellers to market and sell their properties.

     22.  NONDISCRIMINATION.  The parties agree not to  discriminate  unlawfully
against any prospective  buyer because of the race,  creed,  color, sex, marital
status, national origin, familial status, physical or mental handicap,  religion
or ancestry of such person.

     23.  RECOMMENDATION  OF LEGAL  COUNSEL.  By signing this  document,  Seller
acknowledges  that Broker has advised  that this  document has  important  legal
consequences  and has  recommended  consultation  with  legal  and tax or  other
counsel, before signing this contract.

     24. ALTERNATIVE DISPUTE RESOLUTION: MEDIATION. If a dispute arises relating
to this  contract,  and is not  resolved,  the parties  involved in such dispute
(Disputants)  shall  first  proceed  in good  faith  to  submit  the  matter  to
mediation.  The Disputants will jointly appoint an acceptable  mediator and will
share equally in the cost of such mediation.  In the event the entire dispute is
not resolved  within  thirty (30)  calendar  days from the date  written  notice
requesting  mediation is sent by one Disputant to the other(s),  the  mediation,
unless otherwise agreed, shall terminate.  This section shall not alter any date
in this contract, unless otherwise agreed.

<PAGE> 6

     25. ATTORNEY FEES. In case of arbitration or litigation  between Seller and
Broker  in their  respective  capacities,  the  parties  agree  that  costs  and
reasonable attorney fees shall be awarded to the prevailing party.

     26.  MODIFICATION OF THIS LISTING CONTRACT.  No subsequent  modification of
any of the  terms of this  Listing  Contract  shall be valid,  binding  upon the
parties, or enforceable unless made in writing and signed by the parties.

     27.  ENTIRE  AGREEMENT.   This  listing  Contract  constitutes  the  entire
agreement  between the parties  relating  to the subject  hereof,  and any prior
agreements  pertaining  thereto,  whether oral or written,  have been merged and
integrated into this Listing Contract.

     28. ADDITIONAL  PROVISIONS:  (The language of these additional  provisions,
Sections  29 and  30,  has  not  been  approved  by  the  Colorado  Real  Estate
Commission.)

     29. If any of the following parties enters into a purchase contract for the
subject  property  within  the  first  45 days of this  Exclusive  Right to Sell
Agreement,  then "No broker fee will be due."  Broker  shall be due one  quarter
(1/4) of the fee stated herein should one of the following  parties enter into a
purchase  contract during the second 45 days and 100% after 90 days of listing.
These parties are:

         1)   Flatiron Park Company or any of its Agents - Larry Frey
         2)   WW Reynolds Company
         3)   Boulder County

     30.  If the  Property  is sold as part of a  contract  to the  sale of 5600
Flatiron  Parkway  ("CMF-1") by Binswanger,  a Broker retained by the Seller for
the purpose of selling CMF-1,  then  Binswanger  shall be entitled to 50% of the
total commission.

     31.  COPIES OF  AGREEMENT.  This  Listing  Contract is executed in multiple
copies and Seller  acknowledges  receipt  of one copy of this  Listing  Contract
signed by Broker.

     32. COUNTERPARTS. If more than one person is named as a Seller herein, this
Listing  Contract  may be  executed by each  Seller  individually  and when each
Seller has executed a copy of this Listing  Contract such copies taken  together
shall be deemed to be a full and complete contract between the parties.

Accepted:  CB Commercial Real Estate Group,  Inc.       Somatogen, Inc.
           --------------------------------------       ---------------
           Broker                                       Seller

By:        Stan S. O'Dell                               Timothy D. Hoogheem
Address:   1050 - 17th Street, Suite 800                2545 Central Avenue
           Denver, Colorado  80265                      Suite FD1
Phone:     (303) 628-7400                               Boulder, Colorado 80301
                                                        (303) 440-9988




<PAGE> 7



                              DUAL AGENCY ADDENDUM
                          (For In-Company Transactions)


1. AMENDMENT TO AGENCY CONTRACT:  This Dual Agency Addendum is part of a Listing
Contract  dated  September 17, 1997 between the broker named below and its sales
agents  ("Broker") and the undersigned Buyer or Seller. If this addendum is used
with a lease or rental transaction,  the word "Seller" shall mean "Landlord" and
the word "Buyer" shall mean "Tenant". This Addendum will control in the event of
any conflict with the contract to which it is attached.

2. LIMITATION ON BROKER'S AGENCY  OBLIGATIONS.  When acting as the agent for one
party (either Buyer or Seller),  Broker has duties and obligations which include
utmost good faith, loyalty and fidelity to that one principal.  If the principal
consents, however, Broker may act as a Dual Agent when both the Seller and Buyer
have an agency  relationship  with the same  real  estate  company  ("In-Company
Transaction").  A Dual  Agent is a broker  engaged  as a limited  agent for both
Seller and Buyer.  Dual agency creates a conflict of interest  because  Broker's
duties and  obligations of  confidentiality,  full disclosure and loyalty to one
party  conflict  with  those same  duties to the other.  Seller and Buyer may be
vicariously liable for acts of a Dual Agent.

     a. If this  addendum  is  signed  by  Seller,  Broker  will act only as the
exclusive agent for Seller when the Property is shown to a prospective buyer who
has a working  relationship with another licensed real estate company,  but will
act only as a Dual Agent in an In-Company Transaction.

     b. If this  addendum  is  signed  by  Buyer,  Broker  will  act only as the
exclusive  agent for Buyer when  showing  properties  that are not  listed  with
Broker, but will act only as a Dual Agent in an In- Company Transaction.

The remaining  provisions of this addendum describe  significant changes to the
obligations of Broker when acting as a Dual Agent in an In-Company Transaction.

3. MATTERS THAT CAN BE DISCLOSED BY A DUAL AGENT. Except as set forth in Section
4, Broker,  when acting as a Dual Agent,  may disclose  any  information  to one
party that Broker gains from the other party if the  information  is relevant to
the transaction or party.

<PAGE> 8

4. MATTERS THAT CANNOT BE  DISCLOSED BY A DUAL AGENT.  Broker,  when acting as a
Dual Agent,  shall not  disclose  the  following  information  without the prior
consent of Seller and Buyer.
     a. That Buyer is willing to pay more than the purchase  price  offered for
the  property;  
     b. That  Seller is willing to accept  less than the asking price for the 
property;  
     c. What the motivating  factors are for any party buying or  selling  the
property;  
     d. That  Seller or Buyer will agree to financing  terms other than those  
offered;  
     e. Any  material  information about the other party unless either:
        (1) the disclosure is required by law,
        (2) the  disclosure  pertains to adverse  material  facts about Buyer's
            financial ability to perform the terms of the transaction,
        (3) the disclosure pertains to Buyer's intent to occupy the property as
            a principal residence, or (4) failure to disclose such information
            would constitute fraud or dishonest dealing.

5.  ADDITIONAL  PROVISIONS.  Notwithstanding  any  provision  contained  in  the
Agreement,  Somatogen  will  not  be  obligated  to  pay  to CB  Commercial  any
commission,  fee or any other  compensation  including,  but not  limited to the
Commission  Fee,  pursuant to the Listing  Contract or  otherwise,  if Somatogen
sells, leases,  conveys, or in any other way transfers the Property to any party
that was introduced to Somatogen through an entity or individual (such entity or
individual,  an "Investment  Bank") that has a written  agreement with Somatogen
which  obligates  Somatogen to pay a fee to such  Investment  Bank in connection
with a transaction involving Somatogen.


Accepted:    Somatogen, Inc..

Seller:   Timothy D. Hoogheem                        Date:   9/17/97


Broker:  CB Commercial Real Estate Group, Inc.

By:  Stan S. O'Dell                                  Date:   9/18/97




<PAGE> 9



                                   ADDENDUM A
                                       TO
                    EXCLUSIVE RIGHT-TO-SELL LISTING CONTRACT
                                      DATED
                               SEPTEMBER 17, 1997
                                     between
                           SOMATOGEN, INC. ("SELLER")
                                       and
                CB COMMERCIAL REAL ESTATE GROUP, INC. ("BROKER")
                            (the "Listing Contract")


     THIS  ADDENDUM  A (this "Addendum")  is  attached  to and made  part of the
above-referenced Listing Contract. In the event of the existence of any conflict
between the terms set forth in the Listing  Contract  and the terms set forth in
this Addendum,  the terms set forth in this Addendum  shall control.  As used in
this Addendum,  the term "Listing  Contract" shall mean the Listing  Contract as
amended and supplemented by this Addendum.

     1. Price and Terms.  The price for the  Property  set forth in Section 6 of
this  Listing  Contract,  and the other terms for the sale of the  Property  set
forth in Sections 6 through 12 of this Listing  Contract,  are provided  only as
guidelines  to be used by Broker  in  presenting  the  Property  to  prospective
purchasers. Seller reserves the right to change such price and any of such terms
at any time and from time to time.  Seller shall have no  obligation to sell the
Property,  or to enter into any contract for the sale of the  Property,  at such
price or upon any of such terms.

     2. Commission Earned Upon Closing. No commission shall be earned by Broker,
and no commission  shall be payable by Seller,  except upon the  consummation of
the closing of the sale by Seller of the Property in  accordance  with the terms
set forth in this Listing  Contract.  No commission  shall be payable under this
Listing  Contract  unless  and until  the  closing  of the sale of the  Property
occurs,  without  regard to the  reason why a closing  failed to occur.  Without
limiting the generality of the foregoing,  no commission shall be payable in the
event that:  (i) Seller fails to acquire the  Property,  (ii) Seller  refuses or
fails to enter into a contract for sale with any  purchaser,  regardless  of the
terms provided in such contract, (iii) Seller fails or refuses to close the sale
of the Property which Seller has contracted to sell,  regardless of whether such
failure constitutes a breach by Seller under such contract,  or (iv) the closing
of a sale of the Property  fails to occur for any other reason,  including  both
reasons  which are in the  control  of Seller and  reasons  which are not in the
control of Seller.

<PAGE> 10

     3.  Determination  of Co-ops:  Indemnity.  Broker shall be responsible  for
determining  whether any other broker, salesman, or agent is entitled to a co-op
commission  or  other  compensation  on any  sale of the  Property  for  which a
commission is payable to Broker under the terms of this Listing Contract. Broker
shall  indemnify,  defend and hold harmless  Seller from and against any and all
claims for co-op commissions or other compensation  payable to any other broker,
salesman or agent as a result of the Property  hereunder;  provided  that Broker
does not hereby  indemnify  Seller against any claims for  commissions, fees, or
other compensation  payable to any other broker, salesman, or agent arising from
the sale of the  Property to the extent such claims are based upon any  specific
agreement or undertaking made by Seller outside of this Listing Agreement.

     4.  As  Is  Sale.  Broker  shall  market  the  Property  "As  Is,"  without
representation or warranty of any kind from Seller as to any matter whatsoever.

     5. Termination. Seller may terminate this Listing Contract at any time, for
any  reason or no reason,  upon 15 days  written  notice to  Broker,  subject to
Section 14(b)(3) of this Listing Contract.

     6. No  Assignment.  Neither  party hereto shall be permitted to assign this
Listing Contract without the prior written consent of the other party hereto.

     7. Governing Law. The validity and effect of this Listing Contract shall be
determined in accordance with the laws of the State of Colorado.

     8. Facsimile.  Facsimile  signatures are acceptable to and binding upon all
parties to this Listing Contract.