1
                                                                     EXHIBIT 1.1
                                3,000,000 Shares

                           MOLECULAR BIOSYSTEMS, INC.

                                  Common Stock

                             UNDERWRITING AGREEMENT

                                                            May __, 1996

DONALDSON, LUFKIN & JENRETTE
  SECURITIES CORPORATION
OPPENHEIMER & CO., INC.
VECTOR SECURITIES INTERNATIONAL, INC.
  As representatives of the
    several underwriters
    named in Schedule I hereto
  227 Park Avenue
  New York, New York  10172

Dear Sirs:

         Molecular Biosystems, Inc., a Delaware corporation (the "Company")
proposes to issue and sell 3,000,000 shares of its Common Stock, par value $.01
per share (the "Firm Shares"), to the several underwriters named in Schedule I
hereto (the "Underwriters"). The Company also proposes to issue and sell to the
several Underwriters not more than 450,000 additional shares of its Common
Stock, par value $.01 per share (the "Additional Shares"), if requested by the
Underwriters as provided in Section 2 hereof. The Firm Shares and the Additional
Shares are herein collectively called the Shares. The shares of common stock of
the Company to be outstanding after giving effect to the sales contemplated
hereby are hereinafter referred to as the Common Stock.

         1. Registration Statement and Prospectus. The Company has prepared and
filed with the Securities and Exchange Commission (the "Commission") in
accordance with the provisions of the Securities Act of 1933, as amended, and
the rules and regulations of the Commission thereunder (collectively called the
"Act"), a registration statement on Form S-3 including a prospectus relating to
the Shares, which 

                                       1.
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may be amended. The registration statement as amended at the time when it
becomes effective, including any documents and information incorporated or
deemed to be incorporated by reference therein and including a registration
statement (if any) filed pursuant to Rule 462(b) under the Act increasing the
size of the offering registered under the Act and information (if any) deemed to
be part of the registration statement at the time of effectiveness pursuant to
Rule 430A under the Act, is hereinafter referred to as the Registration
Statement; and the prospectus in the form first used to confirm sales of Shares,
including any documents and information incorporated or deemed to be
incorporated by reference therein, is hereinafter referred as the Prospectus.

         2. Agreements to Sell and Purchase. On the basis of the representations
and warranties contained in this Agreement, and subject to its terms and
conditions, the Company agrees to issue and sell, and each Underwriter agrees,
severally and not jointly, to purchase from the Company at a price per share of
$______ (the "Purchase Price") the number of Firm Shares set forth opposite the
name of such Underwriter in Schedule I hereto.

         On the basis of the representations and warranties contained in this
Agreement, and subject to its terms and conditions, the Company agrees to issue
and sell the Additional Shares and the Underwriters shall have the right to
purchase, severally and not jointly, up to 450,000 Additional Shares from the
Company at the Purchase Price. Additional Shares may be purchased solely for the
purpose of covering over-allotments made in connection with the offering of the
Firm Shares. The Underwriters may exercise their right to purchase Additional
Shares in whole or in part from time to time by giving written notice thereof to
the Company within 30 days after the date of this Agreement. You shall give any
such notice on behalf of the Underwriters and such notice shall specify the
aggregate number of Additional Shares to be purchased pursuant to such exercise
and the date for payment and delivery thereof. The date specified in any such
notice shall be a business day (i) no earlier than the Closing Date (as
hereinafter defined), (ii) no later than ten business days after such notice has
been given and (iii) no earlier than two business days after such notice has
been given. If any Additional Shares are to be purchased, each Underwriter,
severally and not jointly, agrees to purchase from the Company the number of
Additional Shares (subject to such adjustments to eliminate fractional shares as
you may determine) which bears the same proportion to the total number of
Additional Shares to be purchased from the Company as the number of Firm Shares
set forth opposite the name of such Underwriter in Schedule I bears to the total
number of Firm Shares.

         The Company hereby agrees to and the Company shall, concurrently with
the execution of this Agreement, deliver an agreement executed by (i) each of
the directors and officers of the Company, and (ii) each stockholder listed on
Schedule II hereto, pursuant to which each such person agrees, not to offer,
sell, contract to sell, grant any option to purchase, or otherwise dispose of
any common stock of the Company or 

                                       2.
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any securities convertible into or exercisable or exchangeable for such common
stock or in any other manner transfer all or a portion of the economic
consequences associated with the ownership of any such common stock, except to
the Underwriters pursuant to this Agreement, for a period of 120 days after the
date of the Prospectus without the prior written consent of Donaldson, Lufkin &
Jenrette Securities Corporation. Notwithstanding the foregoing, during such
period (i) the Company may grant stock options pursuant to the Company's
existing stock option plan and (ii) the Company may issue shares of its common
stock upon the exercise of an option or warrant or the conversion of a security
outstanding on the date hereof.

         3. Terms of Public Offering. The Company is advised by you that the
Underwriters propose (i) to make a public offering of their respective portions
of the Shares as soon after the effective date of the Registration Statement as
in your judgment is advisable and (ii) initially to offer the Shares upon the
terms set forth in the Prospectus.

         4. Delivery and Payment. Delivery to the Underwriters of and payment
for the Firm Shares shall be made at 10:00 A.M., New York City time, on the
third or fourth business day unless otherwise permitted by the Commission
pursuant to Rule 15c6-1 of the Securities Exchange Act of 1934, as amended (the
"Exchange Act")(the "Closing Date") following the date of the initial public
offering, at the offices of Cooley Godward Castro Huddleson & Tatum, 4365
Executive Drive, Suite 1100, San Diego, California 92121 (or such other place as
mutually may be agreed upon). The Closing Date and the location of delivery of
and the form of payment for the Firm Shares may be varied by agreement between
you and the Company.

         Delivery to the Underwriters of and payment for any Additional Shares
to be purchased by the Underwriters shall be made at such place as you shall
designate at 10:00 A.M., New York City time, on the date specified in the
applicable exercise notice given by you pursuant to Section 2 (an "Option
Closing Date"). Any such Option Closing Date and the location of delivery of and
the form of payment for such Additional Shares may be varied by agreement
between you and the Company.

         Certificates for the Shares shall be registered in such names and
issued in such denominations as you shall request in writing not later than two
full business days prior to the Closing Date or an Option Closing Date, as the
case may be. Such certificates shall be made available to you for inspection not
later than 9:30 A.M., New York City time, on the business day next preceding the
Closing Date or the applicable Option Closing Date, as the case may be.
Certificates in definitive form evidencing the Shares shall be delivered to you
on the Closing Date or the applicable Option Closing Date, as the case may be,
with any transfer taxes thereon duly paid by the Company, for the respective
accounts of the several Underwriters, against payment of the Purchase Price
therefor by wire transfer of immediately available funds to the account or
accounts 

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designated in writing to you by the Company at least two business days prior to
the Closing Date or applicable Option Date, as the case may be.

         5. Agreements of the Company. The Company agrees with you:

            (a) To use its best efforts to cause the Registration Statement to
     become effective at the earliest possible time.

            (b) To advise you promptly and, if requested by you, to confirm such
     advice in writing, (i) when the Registration Statement has become effective
     and when any post-effective amendment to it becomes effective, (ii) of any
     request by the Commission for amendments to the Registration Statement or
     amendments or supplements to the Prospectus or for additional information,
     (iii) of the issuance by the Commission of any stop order suspending the
     effectiveness of the Registration Statement or of the suspension of
     qualification of the Shares for offering or sale in any jurisdiction, or
     the initiation of any proceeding for such purposes, and (iv) of the
     happening of any event during the period referred to in Section (e) below
     which makes any statement of a material fact made in the Registration
     Statement or the Prospectus untrue or which requires the making of any
     additions to or changes in the Registration Statement or the Prospectus in
     order to make the statements therein not misleading. If at any time the
     Commission shall issue any stop order suspending the effectiveness of the
     Registration Statement, the Company will make every reasonable effort to
     obtain the withdrawal or lifting of such order at the earliest possible
     time.

            (c) To furnish to you, without charge, four (4) signed copies of the
     Registration Statement as first filed with the Commission and of each
     amendment to it, including all exhibits, and to furnish to you and each
     Underwriter designated by you such number of conformed copies of the
     Registration Statement as so filed and of each amendment to it, without
     exhibits, as you may reasonably request.

            (d) Not to file any amendment or supplement to the Registration
     Statement, whether before or after the time when it becomes effective, or
     to make any amendment or supplement to the Prospectus, of which you shall
     not previously have been advised or to which you shall reasonably object;
     and to prepare and file with the Commission, promptly upon your reasonable
     request, any amendment to the Registration Statement or supplement to the
     Prospectus which may be necessary or advisable in connection with the
     distribution of the Shares by you, and to use its best efforts to cause the
     same to become promptly effective.

            (e) Promptly after the Registration Statement becomes effective, and
     from time to time thereafter for such period as in the opinion of counsel
     for the Underwriters a prospectus is required by law to be delivered in
     connection 

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     with sales by an Underwriter or a dealer, to furnish to each Underwriter
     and dealer as many copies of the Prospectus (and of any amendment or
     supplement to the Prospectus) as such Underwriter or dealer may reasonably
     request.

            (f) If during the period specified in Section (e) any event shall
     occur as a result of which, in the opinion of counsel for the Underwriters,
     it becomes necessary to amend or supplement the Prospectus in order to make
     the statements therein, in the light of the circumstances when the
     Prospectus is delivered to a purchaser, not misleading, or if it is
     necessary to amend or supplement the Prospectus to comply with the Act, the
     rules and regulations of the National Association of Securities Dealers,
     Inc. (the "NASD") or any applicable state securities laws, forthwith to
     prepare and file with the Commission an appropriate amendment or supplement
     to the Prospectus so that the statements in the Prospectus, as so amended
     or supplemented, will not, in the light of the circumstances when it is so
     delivered, be misleading, or so that the Prospectus will comply with law,
     and to furnish to each Underwriter and to such dealers as you shall
     specify, such number of copies thereof as such Underwriter or dealers may
     reasonably request.

            (g) Prior to any public offering of the Shares, to cooperate with
     you and counsel for the Underwriters in connection with the registration or
     qualification of the Shares for offer and sale by the several Underwriters
     and by dealers under the state securities or Blue Sky laws of such
     jurisdictions as you may request, to continue such qualification in effect
     so long as required for distribution of the Shares and to file such
     consents to service of process or other documents as may be necessary in
     order to effect such registration or qualification. The Company shall not
     be required, however, to qualify to do business as a foreign corporation in
     any jurisdiction where it is not now so qualified or to take any action
     that would subject it to general service of process in any jurisdiction
     where it is not now so subject.

            (h) To mail and make generally available to its stockholders as soon
     as reasonably practicable an earnings statement covering a period of at
     least twelve months after the effective date of the Registration Statement
     (but in no event commencing later than 90 days after such date) which shall
     satisfy the provisions of Section 11(a) of the Act, and to advise you in
     writing when such statement has been so made available.

            (i) During the period of five years from the effective date of the
     Registration Statement, to furnish to you as soon as available a copy of
     each report or other publicly available information of the Company mailed
     to the holders of Common Stock or filed with the Commission and such other
     publicly available 

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     information concerning the Company and its subsidiaries as you may
     reasonably request.

            (j) To pay all costs, expenses, fees and taxes arising from (i) the
     preparation, printing, filing and distribution under the Act of the
     Registration Statement (including financial statements and exhibits), each
     preliminary prospectus and all amendments and supplements to any of them
     prior to or during the period specified in Section (e), (ii) the printing
     and delivery of the Prospectus and all amendments or supplements to it
     during the period specified in Section (e), (iii) the printing and delivery
     of this Agreement, the preliminary and supplemental Blue Sky memoranda and
     all other agreements, memoranda, correspondence and other documents printed
     and delivered in connection with the offering of the Shares (including in
     each case any disbursements of counsel for the Underwriters relating to
     such printing and delivery), (iv) the registration or qualification of the
     Shares for offer and sale under the securities or Blue Sky laws of the
     several states (including in each case the fees and disbursements of
     counsel for the Underwriters relating to such registration or qualification
     and memoranda relating thereto), (v) filings and clearance with the NASD in
     connection with the offering, (vi) the listing of the Shares on the New
     York Stock Exchange ("NYSE") and (vii) furnishing such copies of the
     Registration Statement, the Prospectus and all amendments and supplements
     thereto as may be requested for use in connection with the offering or sale
     of the Shares by the Underwriters or by dealers to whom Shares may be sold.

            (k) To use its best efforts to maintain the inclusion of such Common
     Stock on the NYSE for a period of five years after the effective date of
     the Registration Statement.

            (l) To use its reasonable efforts to do and perform all things
     required or necessary to be done and performed under this Agreement by the
     Company prior to the Closing Date or any Option Closing Date, as the case
     may be, and to satisfy all conditions precedent to the delivery of the
     Shares.

         6. Representations and Warranties of the Company. The Company
represents and warrants to each Underwriter that:

            (a) The Registration Statement has become effective; no stop order
     suspending the effectiveness of the Registration Statement is in effect,
     and no proceedings for such purpose are pending before or threatened by the
     Commission.

            (b) (i) Each part of the Registration Statement, when such part
     became effective, did not contain and each such part, as amended or
     supplemented, if applicable, will not contain any untrue statement of a
     material 

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     fact or omit to state a material fact required to be stated therein or
     necessary to make the statements therein not misleading, (ii) the
     Registration Statement and the Prospectus comply and, as amended or
     supplemented, if applicable, will comply in all material respects with the
     Act and (iii) the Prospectus does not contain and, as amended or
     supplemented, if applicable, will not contain any untrue statement of a
     material fact or omit to state a material fact necessary to make the
     statements therein, in the light of the circumstances under which they were
     made, not misleading, except that the representations and warranties set
     forth in this Section (b) do not apply to statements or omissions in the
     Registration Statement or the Prospectus based upon information relating to
     any Underwriter furnished to the Company in writing by such Underwriter
     through you expressly for use therein. The Company acknowledges for all
     purposes under this Agreement (including this Section and Section 7 hereof)
     that the statements set forth in the first and third paragraphs under the
     caption "Underwriting" in the Prospectus constitute the only written
     information furnished to the Company by the Underwriters for use in the
     Registration Statement or the Prospectus or any preliminary prospectus (or
     any amendment or supplement to them).

            (c) Each preliminary prospectus filed as part of the registration
     statement as originally filed or as part of any amendment thereto, or filed
     pursuant to Rule 424 under the Act, and each Registration Statement filed
     pursuant to Rule 462(b) under the Act, if any, complied when so filed in
     all material respects with the Act; and did not contain an untrue statement
     of a material fact or omit to state a material fact required to be stated
     therein or necessary to make the statements therein, in the light of the
     circumstances under which they were made, not misleading.

            (d) The Company and each of its subsidiaries has been duly
     incorporated, is validly existing as a corporation in good standing under
     the laws of its jurisdiction of incorporation and has the corporate power
     and authority to carry on its business as it is currently being conducted
     and to own, lease and operate its properties, and each is duly qualified
     and is in good standing as a foreign corporation authorized to do business
     in each jurisdiction in which the nature of its business or its ownership
     or leasing of property requires such qualification, except where the
     failure to be so qualified or to be in good standing as a foreign
     corporation would not have a material adverse effect on the Company and its
     subsidiaries, taken as a whole.

            (e) All of the outstanding shares of capital stock of, or other
     ownership interests in, each of the Company's subsidiaries have been duly
     authorized and validly issued and are fully paid and non-assessable, and
     are owned by the Company, free and clear of any security interest, claim,
     lien or encumbrance.

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            (f) All the outstanding shares of capital stock of the Company have
     been duly authorized and validly issued and are fully paid, non-assessable
     and not subject to any preemptive or similar rights; and the Shares have
     been duly authorized and, when issued and delivered to the Underwriters
     against payment therefor as provided by this Agreement, will be validly
     issued, fully paid and non-assessable, and the issuance of such Shares will
     not be subject to any preemptive or similar rights.

            (g) The authorized capital stock of the Company, including the
     Common Stock, conforms as to legal matters to the description thereof
     contained in the Prospectus.

            (h) Neither the Company nor any of its subsidiaries is in violation
     of its respective charter or by-laws or, except for any claimed defaults
     described in the Prospectus and except as would not, singly or in the
     aggregate, have a material adverse effect on the business, prospects,
     financial condition or results of operation of the Company and its
     subsidiaries, taken as a whole (a "Material Adverse Effect"), in default in
     the performance of any obligation, agreement or condition contained in any
     bond, debenture, note or any other evidence of indebtedness or in any other
     agreement, indenture or instrument to which the Company or any of its
     subsidiaries is a party or by which it or any of its subsidiaries or their
     respective property is bound.

            (i) The execution, delivery and performance of this Agreement,
     compliance by the Company with all the provisions hereof and the
     consummation of the transactions contemplated hereby will not require any
     consent, approval, authorization or other order of any court, regulatory
     body, administrative agency or other governmental body (except as such may
     be required under the Act, the Exchange Act, the rules and regulations of
     the NASD and the securities or Blue Sky laws of the various states) and
     will not conflict with or constitute a breach of any of the terms or
     provisions of, or a default under, the charter or by-laws of the Company or
     any of its subsidiaries or any agreement, indenture or other instrument to
     which it or any of its subsidiaries is a party or by which it or any of its
     subsidiaries or their respective property is bound, or, except as would
     not, singly or in the aggregate, have a Material Adverse Effect, violate or
     conflict with any laws, administrative regulations or rulings or court
     decrees applicable to the Company, any of its subsidiaries or their
     respective property.

            (j) Except as otherwise set forth in the Prospectus, there are no
     material legal or governmental proceedings pending to which the Company or
     any of its subsidiaries is a party or of which any of their respective
     property is the subject, and, to the Company's knowledge, no such
     proceedings are threatened or contemplated. No contract or document of a
     character required to be described in 

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     the Registration Statement or the Prospectus or to be filed as an exhibit
     to the Registration Statement is not so described or filed as required.

            (k) Neither the Company nor any of its subsidiaries has violated any
     foreign, federal, state or local law or regulation relating to the
     protection of human health and safety, the environment or hazardous or
     toxic substances or wastes, pollutants or contaminants ("Environmental
     Laws"), nor any federal or state law relating to discrimination in the
     hiring, promotion or pay of employees nor any applicable federal or state
     wages and hours laws, nor any provisions of the Employee Retirement Income
     Security Act or the rules and regulations promulgated thereunder, except
     for any such violation which would not, singly or in the aggregate, have a
     Material Adverse Effect, or would otherwise require disclosure in the
     Prospectus pursuant to Item 103 of Regulation S-K under the Act ("Item
     103").

            (l) Except as disclosed in the Prospectus, neither the Company nor
     any of its subsidiaries has received any communication (written or oral),
     whether from a governmental authority, citizens' group, employee or
     otherwise, asserting that the Company or any of its subsidiaries or any
     other person or entity for whom either is or may be liable is not in full
     compliance with any Environmental Laws or permit or authorization required
     under applicable Environmental Laws where such failure to comply fully
     would have a Material Adverse Effect, or would otherwise require disclosure
     in the Prospectus pursuant to Item 103.

            (m) The Company and each of its subsidiaries has such permits,
     licenses, franchises and authorizations of governmental or regulatory
     authorities ("permits"), including, without limitation, under any
     applicable Environmental Laws, as are necessary to own, lease and operate
     its respective properties and to conduct its business, except for any such
     permits the failure to obtain which would not have a Material Adverse
     Effect; the Company and each of its subsidiaries has fulfilled and
     performed all of its material obligations with respect to such permits and,
     except as would not have a Material Adverse Effect, no event has occurred
     which allows, or after notice or lapse of time would allow, revocation or
     termination thereof or results in any other material impairment of the
     rights of the holder of any such permit; and, except as described in the
     Prospectus, such permits contain no restrictions that are materially
     burdensome to the Company or any of its subsidiaries.

            (n) Except as otherwise set forth in the Prospectus or such as are
     not material to the business, prospects, financial condition or results of
     operation of the Company and its subsidiaries, taken as a whole, the
     Company and each of its subsidiaries has good and marketable title, free
     and clear of all liens, claims, 

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     encumbrances and restrictions except liens for taxes not yet due and
     payable, to all property and assets described in the Registration Statement
     as being owned by it. All leases to which the Company or any of its
     subsidiaries is a party are valid and binding and no default by the Company
     or any of its subsidiaries, or to the Company's knowledge, by any lessor,
     has occurred or is continuing thereunder, which might result in any
     material adverse change in the business, prospects, financial condition or
     results of operation of the Company and its subsidiaries, taken as a whole,
     and the Company and its subsidiaries enjoy peaceful and undisturbed
     possession under all such leases to which any of them is a party as lessee
     with such exceptions as do not materially interfere with the use made by
     the Company or such subsidiary.

            (o) In the Company's judgment, the Company and each of its
     subsidiaries maintains reasonably adequate insurance.

            (p) Arthur Andersen LLP are independent public accountants with
     respect to the Company as required by the Act.

            (q) The financial statements, together with related schedules and
     notes forming part of the Registration Statement and the Prospectus (and
     any amendment or supplement thereto), present fairly the consolidated
     financial position, results of operations and changes in financial position
     of the Company and its subsidiaries on the basis stated in the Registration
     Statement at the respective dates or for the respective periods to which
     they apply; such statements and related schedules and notes have been
     prepared in accordance with generally accepted accounting principles
     consistently applied throughout the periods involved, except as disclosed
     therein; and the other financial and statistical information and data set
     forth in the Registration Statement and the Prospectus (and any amendment
     or supplement thereto) is, in all material respects, accurately presented
     and prepared on a basis consistent with such financial statements and the
     books and records of the Company.

            (r) The Company is not an "investment company" or a company
     "controlled" by an "investment company" within the meaning of the
     Investment Company Act of 1940, as amended.

            (s) Except for which waivers have been obtained in writing with
     respect to inclusion in this offering, no holder of any security of the
     Company has any right to require registration of shares of Common Stock or
     any other security of the Company.

            (t) The Company has complied with all provisions of Section 517.075,
     Florida Statutes (Chapter 92-198, Laws of Florida).

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            (u) There are no outstanding subscriptions, rights, warrants,
     options, calls, convertible securities, commitments of sale or liens
     related to or entitling any person to purchase or otherwise to acquire any
     shares of the capital stock of, or other ownership interest in, the Company
     or any subsidiary thereof except (i) as disclosed in the Registration
     Statement and (ii) certain anti-dilution rights of one stockholder, for
     which a waiver has been obtained in writing with respect to the issuance of
     the Shares.

            (v) There are no business relationships or related party
     transactions required to be disclosed in the Registration Statement by Item
     404 of Regulation S-K of the Commission.

            (w) There is (i) no significant unfair labor practice complaint
     pending against the Company or any of its subsidiaries or, to the best
     knowledge of the Company, threatened against any of them, before the
     National Labor Relations Board or any state or local labor relations board,
     and no significant grievance or more significant arbitration proceeding
     arising out of or under any collective bargaining agreement is so pending
     against the Company or any of its subsidiaries or, to the best knowledge of
     the Company, threatened against any of them, and (ii) no significant
     strike, labor dispute, slowdown or stoppage pending against the Company or
     any of its subsidiaries or, to the best knowledge of the Company,
     threatened against it or any of its subsidiaries except for such actions
     specified in clause (i) or (ii) above, which, singly or in the aggregate,
     could not reasonably be expected to have a Material Adverse Effect.

            (x) The Company and each of its subsidiaries maintains a system of
     internal accounting controls sufficient to provide reasonable assurance
     that (i) transactions are executed in accordance with management's general
     or specific authorizations; (ii) transactions are recorded as necessary to
     permit preparation of financial statements in conformity with generally
     accepted accounting principles; (iii) access to assets is permitted only in
     accordance with management's general or specific authorization; and (iv)
     the recorded accountability for assets is compared with the existing assets
     at reasonable intervals and appropriate action is taken with respect to any
     differences.

            (y) All material tax returns required to be filed by the Company and
     each of its subsidiaries in any jurisdiction have been filed, other than
     those filings being contested in good faith, and all material taxes,
     including withholding taxes, penalties and interest, assessments, fees and
     other charges due pursuant to such returns or pursuant to any assessment
     received by the Company or any of its subsidiaries have been paid, other
     than those being contested in good faith and for which adequate reserves
     have been provided. No deficiency or adjustment for any 

                                      11.
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     material taxes has been proposed or assessed against the Company or any of
     its subsidiaries.

            (z) The Company has filed a registration statement pursuant to
     Section 12(b) of the Exchange Act to register the Common Stock, has filed
     an application to list the Shares on the NYSE, and has received
     notification that the listing has been approved, subject to notice of
     issuance of the Shares.

            (aa) The Company and its subsidiaries own, license or possess
     adequate rights to use all material patents, patent rights, licenses,
     inventions, copyrights, know-how (including trade secrets and other
     unpatented and/or unpatentable proprietary or confidential information,
     systems or procedures), trademarks, service marks and trade names presently
     employed by them in connection with the businesses now operated by them,
     and, except as otherwise disclosed in the Prospectus, neither the Company
     nor any of its subsidiaries has received any notice of infringement of or
     conflict with asserted rights of others with respect to any of the
     foregoing, which, singly or in the aggregate, if the subject of any
     unfavorable decision, ruling or finding, would result in a Material Adverse
     Effect.

            (bb) This Agreement has been duly authorized, executed and delivered
     by the Company and constitutes the legal, valid and binding agreement of
     the Company, enforceable against the Company in accordance with its terms
     (except as rights to indemnity and contribution hereunder may be limited by
     applicable law and except as enforcement may be limited by applicable
     bankruptcy, insolvency or similar laws affecting the enforcement of
     creditors' rights generally or by legal and equitable limitations on the
     availability of specific remedies).

            (cc) The Company has full power, authority and legal capacity to
     enter into and perform its obligations under this Agreement and to issue,
     sell and deliver the Shares; and the Company has taken all necessary
     corporate action to authorize the execution and the performance of its
     obligations under this Agreement.

            (dd) No bid or purchase by the Company, and, to the Company's
     knowledge, no bid or purchase that could be attributed to the Company (as a
     result of bids or purchases by an "affiliated purchaser" within the meaning
     of Rule 10b-6 under the Exchange Act for or of the Common Stock, any
     securities of the same class or series as the Common Stock or any
     securities immediately convertible into or exchangeable for or that
     represent any right to acquire Common Stock, is now pending or in progress
     or will have commenced at any time prior to the completion of the
     distribution of the Shares by the Underwriters.

                                      12.
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            (ee) Each of the Company and its subsidiaries is in compliance with
     all laws, ordinances and regulations (domestic and foreign) applicable to
     its properties (whether owned or leased) and its business, as described in
     the Prospectus, except where noncompliance with such laws, ordinances and
     regulations would not, singly or in the aggregate, have a Material Adverse
     Effect.

         7. Indemnification. (a) The Company agrees to indemnify and hold
harmless each Underwriter and each person, if any, who controls any Underwriter
within the meaning of Section 15 of the Act or Section 20 of the Exchange Act,
from and against any and all losses, claims, damages, liabilities and judgments
caused by any untrue statement or alleged untrue statement of a material fact
contained in the Registration Statement or the Prospectus (as amended or
supplemented if the Company shall have furnished any amendments or supplements
thereto) or any preliminary prospectus, or caused by any omission or alleged
omission to state therein a material fact required to be stated therein or
necessary to make the statements therein not misleading, except insofar as such
losses, claims, damages, liabilities or judgments are caused by any such untrue
statement or omission or alleged untrue statement or omission based upon
information relating to any Underwriters furnished in writing to the Company by
or on behalf of any Underwriter through you expressly for use therein; provided,
however, that the foregoing indemnity agreement with respect to any preliminary
prospectus shall not inure to the benefit of any Underwriter from whom the
person asserting any such losses, claims, damages and liabilities and judgments
purchased Shares, or any person controlling such Underwriter, if a copy of the
Prospectus (as then amended or supplemented if the Company shall have furnished
any amendments or supplements thereto) was not sent or given by or on behalf of
such Underwriter to such person, if required by law so to have been delivered,
at or prior to the written confirmation of the sale of the Shares to such
person, and if the Prospectus (as so amended and supplemented) would have cured
the defect giving rise to such loss, claim, damage, liability or judgment.

         (b) In case any action shall be brought against any Underwriter or any
person controlling such Underwriter, based upon any preliminary prospectus, the
Registration Statement or the Prospectus or any amendment or supplement thereto
and with respect to which indemnity may be sought against the Company, such
Underwriter shall promptly notify the Company in writing and the Company shall
assume the defense thereof, including the employment of counsel reasonably
satisfactory to such indemnified party and payment of all fees and expenses of
such counsel. Any Underwriter or any such controlling person shall have the
right to employ separate counsel in any such action and participate in the
defense thereof, but the fees and expenses of such counsel shall be at the
expense of such Underwriter or such controlling person unless (i) the employment
of such counsel shall have been specifically authorized in writing by the
Company, (ii) the Company shall have failed to assume the defense and employ
counsel following prompt notice by such Underwriter or controlling person as
required by this Section 7(b) or (iii) the named parties to any such action
(including any impleaded parties) include 

                                      13.
   14
both such Underwriter or such controlling person and the Company and such
Underwriter or such controlling person shall have been advised by such counsel
that there may be one or more legal defenses available to it which are different
from or additional to those available to the Company (in which case the Company
shall not have the right to assume the defense of such action on behalf of such
Underwriter or such controlling person, it being understood, however, that the
Company shall not, in connection with any one such action or separate but
substantially similar or related actions in the same jurisdiction arising out of
the same general allegations or circumstances, be liable for the fees and
expenses of more than one separate firm of attorneys (in addition to any
reasonable use of local counsel) for all such Underwriters and controlling
persons, which firm shall be designated in writing by Donaldson, Lufkin &
Jenrette Securities Corporation and that all such reasonable fees and expenses
shall be reimbursed as they are incurred). The Company shall not be liable for
any settlement of any such action effected without its written consent but if
settled with the written consent of the Company, the Company agrees to indemnify
and hold harmless any Underwriter and any such controlling person from and
against any loss or liability by reason of such settlement. Notwithstanding the
immediately preceding sentence, if in any case where the fees and expenses of
counsel are at the expense of the indemnifying party and an indemnified party
shall have requested the indemnifying party to reimburse the indemnified party
for such fees and expenses of counsel as incurred, such indemnifying party
agrees that it shall be liable for any settlement of any action effected without
its written consent if (i) such settlement is entered into more than 30 business
days after the receipt by such indemnifying party of the aforesaid request and
(ii) such indemnifying party shall have failed to reimburse the indemnified
party in accordance with such request for reimbursement prior to the date of
such settlement for any reason other than the indemnifying party's dispute in
good faith of the reasonableness of the fees and expenses for which
reimbursement has been requested. No indemnifying party shall, without the prior
written consent of the indemnified party, effect any settlement of any pending
or threatened proceeding in respect of which any indemnified party is or could
have been a party and indemnity could have been sought hereunder by such
indemnified party, unless such settlement includes an unconditional release of
such indemnified party from all liability on claims that are the subject matter
of such proceeding.

         (c) Each Underwriter agrees, severally and not jointly, to indemnify
and hold harmless the Company, its directors, its officers who sign the
Registration Statement and any person controlling the Company within the meaning
of Section 15 of the Act or Section 20 of the Exchange Act, to the same extent
as the foregoing indemnity from the Company to each Underwriter but only with
reference to information relating to such Underwriter furnished in writing by or
on behalf of such Underwriter through you expressly for use in the Registration
Statement, the Prospectus or any preliminary prospectus. In case any action
shall be brought against the Company, any of its directors, any such officer or
any person controlling the Company based on the Registration Statement, the
Prospectus or any preliminary prospectus and in respect of which 

                                      14.
   15
indemnity may be sought against any Underwriter, the Underwriter shall have the
rights and duties given to the Company (except that if the Company shall have
assumed the defense thereof, such Underwriter shall not be required to do so,
but may employ separate counsel therein and participate in the defense thereof
but the fees and expenses of such counsel shall be at the expense of such
Underwriter), and the Company, its directors, any such officers and any person
controlling the Company shall have the rights and duties given to the
Underwriter, by Section 7(b) hereof.

         (d) If the indemnification provided for in this Section 7 is
unavailable to an indemnified party in respect of any losses, claims, damages,
liabilities or judgments referred to therein, then each indemnifying party, in
lieu of indemnifying such indemnified party, shall contribute to the amount paid
or payable by such indemnified party as a result of such losses, claims,
damages, liabilities and judgments (i) in such proportion as is appropriate to
reflect the relative benefits received by the Company on the one hand and the
Underwriters on the other hand from the offering of the Shares or (ii) if the
allocation provided by clause (i) above is not permitted by applicable law, in
such proportion as is appropriate to reflect not only the relative benefits
referred to in clause (i) above but also the relative fault of the Company and
the Underwriters in connection with the statements or omissions which resulted
in such losses, claims, damages, liabilities or judgments, as well as any other
relevant equitable considerations. The relative benefits received by the Company
and the Underwriters shall be deemed to be in the same proportion as the total
net proceeds from the offering (before deducting expenses) received by the
Company, and the total underwriting discounts and commissions received by the
Underwriters, bear to the total price to the public of the Shares, in each case
as set forth in the table on the cover page of the Prospectus. The relative
fault of the Company and the Underwriters shall be determined by reference to,
among other things, whether the untrue or alleged untrue statement of a material
fact or the omission to state a material fact relates to information supplied by
the Company or the Underwriters and the parties' relative intent, knowledge,
access to information and opportunity to correct or prevent such statement or
omission.

         The Company and the Underwriters agree that it would not be just and
equitable if contribution pursuant to this Section 7(d) were determined by pro
rata allocation (even if the Underwriters were treated as one entity for such
purpose) or by any other method of allocation which does not take account of the
equitable considerations referred to in the immediately preceding paragraph. The
amount paid or payable by an indemnified party as a result of the losses,
claims, damages, liabilities or judgments referred to in the immediately
preceding paragraph shall be deemed to include, subject to the limitations set
forth above, any legal or other expenses reasonably incurred by such indemnified
party in connection with investigating or defending any such action or claim.
Notwithstanding the provisions of this Section 7(d), no Underwriter shall be
required to contribute any amount in excess of the amount by which the total
price at which the Shares underwritten by it and distributed to the public were
offered to the 

                                      15.
   16
public exceeds the amount of any damages which such Underwriter has otherwise
been required to pay by reason of such untrue or alleged untrue statement or
omission or alleged omission. No person guilty of fraudulent misrepresentation
(within the meaning of Section 11(f) of the Act) shall be entitled to
contribution from any person who was not guilty of such fraudulent
misrepresentation. The Underwriters' obligations to contribute pursuant to this
Section 7(d) are several in proportion to the respective number of Shares
purchased by each of the Underwriters hereunder and not joint.

         8. Conditions of Underwriters' Obligations. The several obligations of
the Underwriters to purchase the Firm Shares under this Agreement are subject to
the satisfaction of each of the following conditions:

            (a) All the representations and warranties of the Company contained
     in this Agreement shall be true and correct on the Closing Date with the
     same force and effect as if made on and as of the Closing Date.

            (b) The Registration Statement shall have become effective not later
     than 5:00 P.M. (and in the case of a Registration Statement filed under
     Rule 462(b) of the Act, not later than 10:00 P.M.), New York City time, on
     the date of this Agreement or at such later date and time as you may
     approve in writing, and at the Closing Date no stop order suspending the
     effectiveness of the Registration Statement shall have been issued and no
     proceedings for that purpose shall have been commenced or shall be pending
     before or contemplated by the Commission.

            (c) (i) Since the date of the latest balance sheet included in the
     Registration Statement and the Prospectus, there shall not have been any
     material adverse change, or any development involving a prospective
     material adverse change, whether or not arising in the ordinary course of
     business, in the condition, financial or otherwise, or in the earnings,
     affairs or business prospects of the Company, (ii) since the date of the
     latest balance sheet included in the Registration Statement and the
     Prospectus there shall not have been any material change, or any
     development involving a prospective material adverse change, in the capital
     stock or in the long-term debt of the Company from that set forth in the
     Registration Statement and Prospectus, (iii) the Company and its
     subsidiaries shall have no liability or obligation, direct or contingent,
     which is material to the Company and its subsidiaries, taken as a whole,
     other than those reflected in the Registration Statement and the Prospectus
     and (iv) on the Closing Date you shall have received a certificate dated
     the Closing Date, signed by Kenneth J. Widder, M.D. and Gerard A. Wills, in
     their capacities as the Chairman of the Board and Chief Executive Officer
     and the Vice President, Finance and Chief Financial Officer, respectively,
     of the Company, confirming, to the best of their knowledge, the matters set
     forth in Sections (a), (b), and (c) of this Section 8.

                                      16.
   17
            (d) You shall have received on the Closing Date an opinion
     (satisfactory to you and counsel for the Underwriters), dated the Closing
     Date, of Johnson and Colmar, counsel for the Company, to the effect that:

                (i) the Company and each of its subsidiaries has been duly
     incorporated, is validly existing as a corporation in good standing under
     the laws of its jurisdiction of incorporation and has the corporate power
     and authority required to carry on its business as it is currently being
     conducted and to own, lease and operate its properties;

                (ii) the Company and each of its subsidiaries is duly qualified
     and is in good standing as a foreign corporation authorized to do business
     in each jurisdiction in which the nature of its business or its ownership
     or leasing of property requires such qualification, except where the
     failure to be so qualified would not have a material adverse effect on the
     Company and its subsidiaries, taken as a whole;

                (iii) all of the outstanding shares of capital stock of, or
     other ownership interests in, each of the Company's subsidiaries have been
     duly and validly authorized and issued and are fully paid and
     non-assessable, and are owned by the Company and, to such counsel's
     knowledge, free and clear of any security interest, claim, lien,
     encumbrance or adverse interest of any nature;

                (iv) all the outstanding shares of Common Stock have been duly
     authorized and validly issued and are fully paid, non-assessable and, to
     such counsel's knowledge, not subject to any preemptive or similar rights;

                (v) the Shares have been duly authorized, and when issued and
     delivered to the Underwriters against payment therefor as provided by this
     Agreement, will have been validly issued and will be fully paid and
     non-assessable, and the issuance of such Shares is not subject to any
     statutory preemptive rights or, to such counsel's knowledge, any similar
     contractual rights that have not been waived in writing;

                (vi) this Agreement has been duly authorized, executed and
     delivered by the Company and is a valid and binding agreement of the
     Company enforceable in accordance with its terms (except as rights to
     indemnity and contribution hereunder may be limited by applicable law and
     except as enforcement may be limited by applicable bankruptcy, insolvency
     or similar laws affecting the enforcement of creditors' rights generally or
     by legal and equitable limitations on the availability of specific
     remedies);

                                      17.
   18
                (vii) the authorized capital stock of the Company, including the
     Common Stock, conforms in all material respects as to legal matters to the
     description thereof contained in the Prospectus;

                (viii) the Registration Statement has become effective under the
     Act, and to the best of such counsel's knowledge, after due inquiry, no
     stop order suspending its effectiveness has been issued and no proceedings
     for that purpose are pending before or contemplated by the Commission;

                (ix) such contracts and documents as are summarized in the
     Prospectus are fairly summarized in all material respects;

                (x) the statements under the captions "Shares Eligible for
     Future Sale" and "Description of Capital Stock" in the Prospectus and Item
     15 of Part II of the Registration Statement, insofar as such statements
     constitute a summary of legal matters, documents or proceedings referred to
     therein, fairly present the information called for with respect to such
     legal matters, documents and proceedings;

                (xi) neither the Company nor any of its subsidiaries is in
     violation of its respective charter or by-laws and, to such counsel's
     knowledge, neither the Company nor any of its subsidiaries is in default in
     the performance of any obligation, agreement or condition contained in any
     bond, debenture, note or any other evidence of indebtedness or, except for
     claimed defaults as described in the Prospectus, in any other agreement,
     indenture or instrument material to the conduct of the business of the
     Company and its subsidiaries, taken as a whole, to which the Company or any
     of its subsidiaries is a party or by which it or any of its subsidiaries or
     their respective property is bound;

                (xii) the execution, delivery and performance of this Agreement
     by the Company, compliance by the Company with all the provisions hereof
     and the consummation of the transactions contemplated hereby will not
     require any consent, approval, authorization or other order of any court,
     regulatory body, administrative agency or other governmental body (except
     as such may be required under the Act, the Exchange Act, the rules and
     regulations of the NASD or other securities or Blue Sky laws) and will not
     conflict with or constitute a breach of any of the terms or provisions of,
     or a default under, the charter or by-laws of the Company or any of its
     subsidiaries, or any agreement, indenture or other instrument known to such
     counsel to which the Company or any of its subsidiaries is a party or by
     which the Company or any of its subsidiaries or their respective properties
     are bound, or to such counsel's knowledge, violate or conflict with any
     laws, administrative regulations or rulings or court decrees applicable to

                                      18.
   19
     the Company or any of its subsidiaries or their respective properties,
     which violations or conflicts would have a Material Adverse Effect;

                (xiii) to such counsel's knowledge, there is no legal or
     governmental proceeding pending or threatened to which the Company or any
     of its subsidiaries is a party or to which any of their respective property
     is subject which is required to be described in the Registration Statement
     or the Prospectus and is not so described, nor to the knowledge of such
     counsel is there any contract or other document which is required to be
     described in the Registration Statement or the Prospectus or is required to
     be filed as an exhibit to the Registration Statement which is not described
     or filed as required;

                (xiv) to such counsel's knowledge, the Company and each of its
     subsidiaries has such permits, licenses, franchises and authorizations of
     governmental or regulatory authorities ("permits"), including, without
     limitation, under any applicable Environmental Laws, as are necessary to
     own, lease and operate its respective properties and to conduct its
     business in the manner described in the Prospectus; to such counsel's
     knowledge, the Company and each of its subsidiaries has fulfilled and
     performed all of its material obligations with respect to such permits and
     no event has occurred which allows, or after notice or lapse of time would
     allow, revocation or termination thereof or results in any other material
     impairment of the rights of the holder of any such permit;

                (xv) the Company is not an "investment company" or a company
     "controlled" by an "investment company" within the meaning of the
     Investment Company Act of 1940, as amended;

                (xvi) to the best of such counsel's knowledge, after due
     inquiry, except for which waivers have been obtained in writing with
     respect to inclusion in this offering, no holder of any security of the
     Company has any right to require registration of shares of Common Stock or
     any other security of the Company;

                (xvii) Each document filed pursuant to the Exchange Act and
     incorporated by reference in the Prospectus, at the time it was filed or
     last amended (other than financial statements and related schedules and
     financial and statistical data, as to which such counsel need express no
     opinion), appeared on its face to be appropriately responsive in all
     material respects to the requirements of the Exchange Act.

         In addition, such counsel shall state that, although such counsel has
not passed upon and does not assume any responsibility for the accuracy,
completeness or fairness of the statements contained in the Registration
Statement or Prospectus (except as specified elsewhere in such counsel's
opinion) and has not made any independent 

                                      19.
   20
verification or check of such statements, such counsel has participated in the
preparation of the Registration Statement and the Prospectus and in conferences
with certain officers and representatives of the Company and representatives of
the Underwriters and the Underwriters' counsel at which the contents of the
Registration Statement, the Prospectus and related matters were discussed; and
that on the basis of this participation and subject to the qualification noted,
(i) such counsel believes that the Registration Statement and the Prospectus
(except for financial statements and related schedules and financial and
statistical data ("financial information"), as to which such counsel need
express no opinion) comply as to form in all material respects with the Act and
the related rules and regulations of the Commission, and (ii) no facts have come
to the attention of such counsel which causes such counsel to believe that (A)
as of the effective date of the Registration Statement, the Registration
Statement (excluding financial information, as to which such counsel need
express no belief) contained an untrue statement of a material fact or omitted
to state a material fact required to be stated therein or necessary to make the
statements therein not misleading or that (B) as of the date of the Prospectus
and as of the date of such counsel's opinion, the Prospectus (excluding
financial information, as to which such counsel need express no belief)
contained an untrue statement of a material fact or omitted to state a material
fact required to be stated therein or necessary to make the statements therein,
in light of the circumstances in which they were made, not misleading.

            (e) You shall have received on the Closing Date the opinion
     (satisfactory to you and counsel for the Underwriters), dated the Closing
     Date, of Morrison & Foerster, patent counsel for the Company, to the effect
     that:

                (i) such counsel is familiar with the technology used by the
     Company in its business and the manner of its use thereof and has read the
     Registration Statement and the Prospectus, including particularly the
     portions of the Registration Statement and the Prospectus referring to
     patents, trade secrets, trademarks, service marks or other proprietary
     information or materials; and

                (ii) such counsel has no reason to believe that the Registration
     Statement or the Prospectus (A) contains any untrue statement of a material
     fact with respect to patents, trade secrets, trademarks, service marks or
     other proprietary information or materials owned or used by the Company, or
     the manner of its use thereof, or any allegation on the part of any person
     that the Company is infringing any patent rights, trade secrets,
     trademarks, service marks or other proprietary information or materials of
     any such person or (B) omits to state any material fact relating to
     patents, trade secrets, trademarks, service marks or other proprietary
     information or materials owned or used by the Company, or the manner of its
     use thereof, or any allegation of which such counsel has 

                                      20.
   21
     knowledge, that is required to be stated in the Registration Statement or
     the Prospectus or is necessary to make the statements therein not
     misleading.

            (f) You shall have received on the Closing Date the opinion
     (satisfactory to you and counsel for the Underwriters), dated the Closing
     Date, of _________________, regulatory counsel for the Company, to the
     effect that the statements under the captions "Risk Factors -- Government
     Regulation," "Risk Factors -- Uncertainty of Third Party Reimbursement,"
     "Business -- Government Regulation" and "Business -- Third Party
     Reimbursement" in the Prospectus, insofar as such statements constitute a
     summary of legal matters, documents or proceedings referred to therein,
     fairly present the information called for with respect to such legal
     matters, documents and proceedings.

            The opinions of Johnson and Colmar, Morrison & Foerster described in
Sections (d), (e) and (f) above shall be rendered to you at the request of the
Company and shall so state therein.

            (g) You shall have received on the Closing Date an opinion, dated
     the Closing Date, of Cooley Godward Castro Huddleson & Tatum ("Cooley
     Godward"), counsel for the Underwriters, in form and substance reasonably
     satisfactory to you.

            (h) You shall have received letters on and as of the date hereof as
     well as on and as of the Closing Date (in the latter case constituting an
     affirmation of the statements set forth in the former, based on limited
     procedures), in form and substance satisfactory to you, from Arthur
     Andersen LLP, independent public accountants, with respect to the financial
     statements and certain financial information contained in the Registration
     Statement and the Prospectus.

            (i) The Company shall have delivered to you the agreements specified
     in Section 2 hereof.

            (j) At the Closing Date, the Shares shall have been approved for
     listing on the New York Stock Exchange, subject to official notice of
     issuance.

            (k) Cooley Godward shall have been furnished with such documents and
     opinions as they may reasonably require for the purpose of enabling them to
     review or pass upon the matters referred to in this Section 8 and in order
     to evidence the accuracy, completeness or satisfaction in all material
     respects of any of the representations, warranties or conditions herein
     contained.

            (l) Prior to the Closing Date, the Company shall have furnished to
     you or caused to be furnished to you such further information,
     certificates, opinions and documents as you may reasonably request.

                                      21.
   22
            (m) The Company shall not have failed at or prior to the Closing
     Date to perform or comply with any of the agreements herein contained and
     required to be performed or complied with by the Company at or prior to the
     Closing Date.

The several obligations of the Underwriters to purchase any Additional Shares
hereunder are subject to satisfaction on and as of each Option Closing Date of
the conditions set forth in Sections (a) through (m) except that the opinions
called for in Sections (d), (e), (f) and (g) and the letter referred to in
Section (h) shall be revised to reflect the sale of the Additional Shares.

         9. Effective Date of Agreement and Termination. This Agreement shall
become effective upon the later of (i) execution of this Agreement and (ii) when
notification of the effectiveness of the Registration Statement has been
released by the Commission.

         This Agreement may be terminated at any time prior to the Closing Date
by you by written notice to the Company if any of the following has occurred:
(i) since the respective dates as of which information is given in the
Registration Statement and the Prospectus, any adverse change or development
involving a prospective material adverse change in the condition, financial or
otherwise, of the Company and its subsidiaries, taken as a whole, or the
earnings, affairs, or business prospects of the Company and its subsidiaries,
taken as a whole, whether or not arising in the ordinary course of business,
which would, in your judgment, make it impracticable to market the Shares on the
terms and in the manner contemplated in the Prospectus, (ii) any outbreak or
escalation of hostilities or other national or international calamity or crisis
or change in economic conditions or in the financial markets of the United
States or elsewhere that, in your judgment, is material and adverse and would,
in your judgment, make it impracticable to market the Shares on the terms and in
the manner contemplated in the Prospectus, (iii) the suspension or material
limitation of trading in securities on the New York Stock Exchange, the American
Stock Exchange or the Nasdaq Stock Market (National Market), (iv) the enactment,
publication, decree or other promulgation of any federal or state statute,
regulation, rule or order of any court or other governmental authority which in
your opinion materially and adversely affects, or will materially and adversely
affect, the business or operations of the Company or any subsidiary, (v) the
declaration of a banking moratorium by either federal or New York State
authorities or (vi) the taking of any action by any federal, state or local
government or agency in respect of its monetary or fiscal affairs which in your
judgment has a material adverse effect on the financial markets in the United
States.

         If on the Closing Date or on an Option Closing Date, as the case may
be, any one or more of the Underwriters shall fail or refuse to purchase the
Firm Shares or Additional Shares, as the case may be, which it or they have
agreed to purchase 

                                      22.
   23
hereunder on such date and the aggregate number of Firm Shares or Additional
Shares, as the case may be, which such defaulting Underwriter or Underwriters,
as the case may be, agreed but failed or refused to purchase is not more than
one-tenth of the total number of Shares to be purchased on such date by all
Underwriters, each non-defaulting Underwriter shall be obligated severally, in
the proportion which the number of Firm Shares set forth opposite its name in
Schedule I bears to the total number of Firm Shares which all the non-defaulting
Underwriters, as the case may be, have agreed to purchase, or in such other
proportion as you may specify, to purchase the Firm Shares or Additional Shares,
as the case may be, which such defaulting Underwriter or Underwriters, as the
case may be, agreed but failed or refused to purchase on such date; provided
that in no event shall the number of Firm Shares or Additional Shares, as the
case may be, which any Underwriter has agreed to purchase pursuant to Section 2
hereof be increased pursuant to this Section 9 by an amount in excess of
one-ninth of such number of Firm Shares or Additional Shares, as the case may
be, without the written consent of such Underwriter. If on the Closing Date or
on an Option Closing Date, as the case may be, any Underwriter or Underwriters
shall fail or refuse to purchase Firm Shares, or Additional Shares, as the case
may be, and the aggregate number of Firm Shares or Additional Shares, as the
case may be, with respect to which such default occurs is more than one-tenth of
the aggregate number of Shares to be purchased on such date by all Underwriters
and arrangements satisfactory to you and the Company for purchase of such Shares
are not made within 48 hours after such default, this Agreement will terminate
without liability on the part of any non-defaulting Underwriter and the Company.
In any such case which does not result in termination of this Agreement, either
you or the Company shall have the right to postpone the Closing Date or the
applicable Option Closing Date, as the case may be, but in no event for longer
than seven days, in order that the required changes, if any, in the Registration
Statement and the Prospectus or any other documents or arrangements may be
effected. Any action taken under this paragraph shall not relieve any defaulting
Underwriter from liability in respect of any default of any such Underwriter
under this Agreement.

         10. Miscellaneous. Notices given pursuant to any provision of this
Agreement shall be addressed as follows: (a) if to the Company, to Molecular
Biosystems, Inc., 10030 Barnes Canyon Road, San Diego, California 92121,
Attention: Vice President, Legal Affairs and General Counsel (with a required
copy to Johnson and Colmar, 300 South Wacker Drive, Chicago, Illinois 60606,
Attention: Craig P. Colmar, Esq.), and (b) if to any Underwriter or to you, to
you c/o Donaldson, Lufkin & Jenrette Securities Corporation, 227 Park Avenue,
New York, New York 10172, Attention: Syndicate Department (with a required copy
to Cooley Godward Castro Huddleson & Tatum, 4365 Executive Drive, Suite 1100,
San Diego, California 92121, Attention: Eric J. Loumeau, Esq.), or in any case
to such other address as the person to be notified may have requested in
writing.

                                      23.
   24
         The respective indemnities, contribution agreements, representations,
warranties and other statements of the Company, its officers and directors and
of the several Underwriters set forth in or made pursuant to this Agreement
shall remain operative and in full force and effect, and will survive delivery
of and payment for the Shares, regardless of (i) any investigation, or statement
as to the results thereof, made by or on behalf of any Underwriter or by or on
behalf of the Company, the officers or directors of the Company or any
controlling person of the Company, (ii) acceptance of the Shares and payment for
them hereunder and (iii) termination of this Agreement.

         If this Agreement shall be terminated by the Underwriters because of
any failure or refusal on the part of the Company to comply with the terms or to
fulfill any of the conditions of this Agreement, the Company agrees to reimburse
the several Underwriters for all out-of-pocket expenses (including the fees and
disbursements of counsel) reasonably incurred by them.

         Except as otherwise provided, this Agreement has been and is made
solely for the benefit of and shall be binding upon the Company, the
Underwriters, any controlling persons referred to herein and their respective
successors and assigns, all as and to the extent provided in this Agreement, and
no other person shall acquire or have any right under or by virtue of this
Agreement. The term "successors and assigns" shall not include a purchaser of
any of the Shares from any of the several Underwriters merely because of such
purchase.

         This Agreement shall be governed and construed in accordance with the
laws of the State of New York.

         This Agreement may be signed in various counterparts which together
shall constitute one and the same instrument.

                                      24.
   25
         Please confirm that the foregoing correctly sets forth the agreement
between the Company and the several Underwriters.


                                                  Very truly yours,

                                                  MOLECULAR BIOSYSTEMS, INC.


                                                  By____________________________
                                                        Title:

DONALDSON, LUFKIN & JENRETTE
  SECURITIES CORPORATION
OPPENHEIMER & CO., INC.
VECTOR SECURITIES INTERNATIONAL, INC.
Acting severally on behalf of
  themselves and the several
  Underwriters named in
  Schedule I hereto
By DONALDSON, LUFKIN & JENRETTE
      SECURITIES CORPORATION

By__________________________




                                      25.
   26
                                   SCHEDULE I





                                                              Number of Firm
Underwriters                                              Shares to be Purchased
- ------------                                              ----------------------
                                                       
Donaldson, Lufkin & Jenrette
  Securities Corporation

Oppenheimer & Co., Inc.

Vector Securities International, Inc.
                                                          ----------------------
     Total                                                      3,000,000

   27
                                   SCHEDULE II


Mallinckrodt Medical, Inc.