<PAGE>

                                                                  EXHIBIT 10.49

                                SUBLEASE (BUILDING #3)

     This Sublease is made as of February 16, 1997, by and between ComStream
Corporation, a Delaware corporation ("Sublessor"), and Molecular Biosystems,
Inc., a Delaware corporation ("Subtenant"), with respect to the approximately
30,097 square-foot building located at 10070 Barnes Canyon Road in San Diego,
California, as depicted in the attached Exhibit "A" (the "Property").
     
     1.  LEASING AND TERM.  Subject to the conditions set forth below, Sublessor
leases to Subtenant, and Subtenant hires from Sublessor, the Property for the
term (the "Term") commencing on March 1, 1998 and expiring on February 14, 2000
(the "Expiration Date"), unless earlier terminated in accordance with this
Sublease; provided, however, Tenant may not take possession of the 10,295
square-foot portion of laboratory within the Property depicted as Space "B" on
the attached EXHIBIT "B" until April 16, 1998.  
     
     2.  MASTER LEASE.  Sublessor's interest in the Property is held under the
Triple Net Lease (Single Tenant Building) dated as of May 31, 1989, by and
between K L Sorrento Associates, predecessor-in-interest to Spieker Properties
("Master Lessor"), as landlord, and Sublessor, as tenant, as amended by the 1st
Amendment to Lease dated as of May 31, 1989, the Second Amendment to Lease dated
as of April 1, 1991, the Third Amendment to Lease dated as of February 15, 1994,
and the Fourth Amendment to Lease dated as of February 24, 1995 (the "Master
Lease"), a copy of which is attached to this Sublease as EXHIBIT C (along with
certain material correspondence and estoppel certificates).  This Sublease is a
sublease under and subject and subordinate to the terms and conditions of the
Master Lease.  Subtenant may not violate any provision of the Master Lease.  The
following provisions of the Master Lease are repeated and incorporated into this
Sublease as if originally set forth in this Sublease, but substituting each
instance of the word "Lessor" with "Sublessor", each instance of "Lessee" with
"Subtenant" and each instance of "Premises" with "Property":  Sections 4.2, 6.1,
7, 8.2, and 8.3, and Articles 11 through 27, 29, 33 through 43, and 46 through
50 (except that in Section 50.5, the reference to "40.54 percent" is replaced
with "32.2 percent"); as well as Article 11 of the Second Amendment to Lease,
and Articles 7, 8, 10, 12, and 13 of the Third Amendment to Lease.  If the
Master Lease is terminated, then this Sublease is automatically terminated on
the same date of such termination and Sublessor has no further obligations under
this Sublease other than to return any unearned paid rent to Subtenant.  The
obligation of Sublessor under this Sublease is expressly conditioned upon
Sublessor obtaining Master Lessor's written consent to this Sublease.

     3.  RENT.  Subtenant shall pay Sublessor as basic monthly rent under this
Sublease, excluding triple net charges, in advance, $28,592.15, except that
basic monthly rent for the first two month period of March 1, 1998 through April
30, 1998 is only $42,514.00, half of which is immediately payable, the other
half of which is due on April 1, 1998.  On March 1, 1999, and for the remainder
of the Term, basic monthly rent increases to $29,449.91.  In order to facilitate
Sublessor paying its obligations under the Master Lease in a timely fashion, all
payments required of Subtenant under this Sublease are due, and must be paid,
five business days before they are due from Sublessor under the Master Lease.  
     
     4.  SECURITY DEPOSIT AND SECURITY INTEREST.  Subtenant shall immediately
deposit with Sublessor cash in the amount of $28,592.15 (the "Security Deposit")
to secure the performance by Subtenant of its obligations under this Sublease,
including Subtenant's obligations (i) to pay rent, (ii) to repair damages caused
by Subtenant or Subtenant's agents, employees, contractors, licensees, or
invitees (collectively, "Subtenant's Invitees"), (iii) to clean the Premises on
the termination of this Sublease, and (iv) to remedy any other defaults by
Subtenant in the performance of any of its obligations under this 


                                       1

<PAGE>

Sublease. Subtenant shall promptly pay to Sublessor the amount necessary to 
replenish any portion of the Security Deposit so used by Sublessor.  
Following the Expiration Date or earlier termination of this Sublease, and 
within the time frame required by applicable law, Sublessor shall deliver to 
Subtenant, at Subtenant's last known address, any portion of the Security 
Deposit not used by Sublessor in accordance with this paragraph, along with 
an itemized statement of deductions made from the Security Deposit.  
Sublessor may commingle the Security Deposit with Sublessor's other funds and 
Sublessor will not pay interest on such Security Deposit to Subtenant. 

     5.  SECURITY MEASURES.  Subtenant acknowledges (i) that the rent does not
include the cost of any security measures for any portion of the Project (ii)
that Sublessor has no obligation to provide any security measures, (iii) that
Sublessor has made no representation to Subtenant regarding the safety or
security of the Project, and (iv) that Subtenant is solely responsible for
providing any security it deems necessary to protect itself, its property, and
Subtenant's invitees in, on, or about the Project.

     6.  "AS IS" CONDITION OF PROPERTY.  Subtenant has inspected the Property to
the extent it deems appropriate and hereby accepts the Property in its "AS IS"
condition or status as of the date of this Sublease and acknowledges that in
doing so it has relied solely on Subtenant's inspection of the Property, its
analysis of the Master Lease and Master Lessor and the opinions and advice
concerning the Premises of consultants engaged by Subtenant.  Subtenant
acknowledges that Sublessor has made no representation or warranty regarding the
Property or the Master Lease or any other matter whatsoever relating to this
Sublease or the Property, and Sublessor disclaims any implied representation or
warranty.

     7.  BROKERS.  Subtenant and Sublessor each represents that no real estate
broker, agent, finder, or other person is responsible for bringing about or
negotiating this Sublease and Subtenant has not dealt with any real estate
broker, agent, finder, or other person, relative to this Sublease in any manner,
other than CB Commercial and The Irving Hughes Group, Inc.  Sublessor is
responsible for paying CB Commercial's commission of $35,025.00, which is to be
split evenly between CB Commercial and The Irving Hughes Group, Inc.

     8.  NOTICES.  Each notice and other communication required or permitted to
be given under this Agreement ("Notice") must be in writing.  Notice is duly
given to another party upon:  (a) hand delivery to the other party, (b) receipt
by the other party when sent by facsimile to the address and number for such
party set forth below (provided, however, that the Notice is not effective
unless a duplicate copy of the facsimile Notice is promptly given by one of the
other methods permitted under this paragraph), (c) three business days after the
Notice has been deposited with the United States postal service as first class
certified mail, return receipt requested, postage prepaid, and addressed to the
party as set forth below, or (d) the next business day after the Notice has been
deposited with a reputable overnight delivery service, postage prepaid,
addressed to the party as set forth below with next-business-day delivery
guaranteed, provided that the sending party receives a confirmation of delivery
from the delivery-service-provider.

If To Sublessor:              ComStream Corporation
                              10180 Barnes Canyon Road
                              San Diego, CA 92121
                              Facsimile:  (619) 657-5702
                              Attn:  General Counsel 

  with a copy to:             Solomon Ward Seidenwurm & Smith, LLP
                              401 B Street, Suite 1200
                              San Diego, CA 92101


                                       2

<PAGE>

                              Attn:  Jeffrey A. Schneider, Esq.
                              Facsimile:  (619) 231-4755

If To Subtenant:              Molecular Biosystems, Inc. 
                              10030 Barnes Canyon Road
                              San Diego, CA 92121
                              Attn:  Gerard A. Wills
                              Facsimile:   (619) 824-2395

Each party shall make a reasonable, good faith effort to ensure that it will
accept or receive Notices to it that are given in accordance with this
paragraph.  A party may change its address for purposes of this paragraph by
giving the other party(ies) written notice of a new address in the manner set
forth above.
     
     9.  MISCELLANEOUS.  This Sublease may be executed in counterparts, each of
which is deemed an original and all of which together constitute one document. 
All exhibits attached to and referenced in this Sublease are incorporated into
this Sublease.  This Sublease may be modified only by a contract in writing
executed by the party to this Sublease against whom enforcement of the
modification is sought.  This Sublease and all documents specifically referred
to and executed in connection with this Sublease:  (a) contain the entire and
final agreement of the parties to this Sublease with respect to the subject
matter of this Sublease, and (b) supersede all negotiations, stipulations,
understandings, agreements, representations and warranties, if any, with respect
to such subject matter, which precede or accompany the execution of this
Sublease.  Whenever the context so requires in this Sublease, all words used in
the singular may include the plural (and vice versa) and the word "person"
includes a natural person, a corporation, a firm, a partnership, a joint
venture, a trust, an estate or any other entity.  The terms "includes" and
"including" do not imply any limitation.  No remedy or election under this
Sublease is exclusive, but rather, to the extent permitted by applicable law,
each such remedy and election is cumulative with all other remedies at law or in
equity.  Each party to this Sublease and its legal counsel have reviewed and
revised this Sublease.  The rule of construction that ambiguities are to be
resolved against the drafting party or in favor of the party receiving a
particular benefit under an agreement may not be employed in the interpretation
of this Sublease or any amendment to this Sublease.  Nothing in this Sublease is
intended to confer any rights or remedies on any person or entity other than the
parties to this Sublease and their respective successors-in-interest and
permitted assignees.


SUBLESSOR:                 COMSTREAM CORPORATION, a Delaware corporation


                           By:  /s/Jamie Crichton                      
                                -------------------------------
                                its:  Vice President & CFO   

SUBTENANT:                 MOLECULAR BIOSYSTEMS, INC., a Delaware 
                           corporation
                              

                           By:  /s/ Thomas E. Jurgensen
                                -------------------------------
                                its: Vice President - Legal and General Counsel


                                       3

<PAGE>




                                     EXHIBIT A


                             THE PROPERTY - BUILDING #3





                                       4


<PAGE>


                                     EXHIBIT B

                                    MASTER LEASE





                                       5