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                                                                   Exhibit 10.58

          THIS WARRANT AND THE SHARES OF COMMON STOCK ISSUED UPON ITS
                  EXERCISE ARE SUBJECT TO THE RESTRICTIONS ON
                   TRANSFER SET FORTH IN SECTION 4 OF THIS WARRANT
          -----------------------------------------------------------

Warrant No. BBH-1                                       Number of Shares: 20,000
                                                         (subject to adjustment)
Date of Issuance:  April 3, 2000

                               NMT MEDICAL, INC.
                               -----------------

                         Common Stock Purchase Warrant
                         -----------------------------

                          (Void after April 3, 2005)

     NMT Medical, Inc., a Delaware corporation (the "Company"), for value
received, hereby certifies that Brown Brothers Harriman & Co., or its registered
assigns (the "Registered Holder"), is entitled, subject to the terms and
conditions set forth below, to purchase from the Company, at any time or from
time to time on or after the date of issuance and on or before 5:00 p.m. (Boston
time) on April 3, 2005, 20,000 shares of Common Stock, $.001 par value per
share, of the Company, at a purchase price of $4.9375 per share.  The shares
purchasable upon exercise of this Warrant, and the purchase price per share,
each as adjusted from time to time pursuant to the provisions of this Warrant,
are hereinafter referred to as the "Warrant Shares" and the "Purchase Price,"
respectively.

     1.  Exercise.

         (a)  This Warrant may be exercised by the Registered Holder, in whole
or in part, by surrendering this Warrant, with the purchase form appended hereto
as Exhibit I duly executed by the Registered Holder or by the Registered
Holder's duly authorized attorney, at the principal office of the Company, or at
such other office or agency as the Company may designate, accompanied by payment
in full, in lawful money of the United States, of the Purchase Price payable in
respect of the number of Warrant Shares purchased upon such exercise.

         (b)  The Registered Holder may, at its option, elect to pay some or
all of the Purchase Price payable upon an exercise of this Warrant by cancelling
a portion of this Warrant exercisable for such number of Warrant Shares as is
determined by dividing (i) the total Purchase Price payable in respect of the
number of Warrant Shares being purchased upon such exercise by (ii) the excess
of the Fair Market Value per share of Common Stock (as defined below) as of the
Exercise Date (as defined in subsection 1(c) below) over the Purchase Price per
share. If the Registered Holder wishes to exercise this Warrant pursuant to this
method of payment with respect to the maximum number of Warrant Shares
purchasable pursuant to this method, then the number of Warrant Shares so
purchasable shall be equal to the total number of Warrant Shares, minus the
product obtained by multiplying (x) the total number of Warrant Shares by (y) a
fraction, the numerator of which shall be the Purchase Price per share and the
denominator of


<PAGE>

which shall be the Fair Market Value per share of Common Stock as of the
Exercise Date. The Fair Market Value per share of Common Stock shall be
determined as follows:

          (i)    If the Common Stock is listed on a national securities
exchange, the Nasdaq National Market or another nationally recognized trading
system as of the Exercise Date, the Fair Market Value per share of Common Stock
shall be deemed to be the average of the high and low reported sale prices per
share of Common Stock thereon on the trading day immediately preceding the
Exercise Date; or, if no such price is reported on such date, the average of
the high and low reported sale prices per share of Common Stock on the next
preceding day with a trade (but not more than five trading days) (provided that
if no such price is reported on such day, the Fair Market Value per share of
Common Stock shall be determined pursuant to clause (ii)).

          (ii)   If the Common Stock is not listed on a national securities
exchange, the Nasdaq National Market or another nationally recognized trading
system as of the Exercise Date, the Fair Market Value per share of Common Stock
shall be deemed to be the amount most recently determined by the Board of
Directors to represent the fair market value per share of the Common Stock
(including without limitation a determination for purposes of granting Common
Stock options or issuing Common Stock under an employee benefit plan of the
Company); and, upon request of the Registered Holder, the Board of Directors (or
a representative thereof) shall promptly notify the Registered Holder of the
Fair Market Value per share of Common Stock. Notwithstanding the foregoing, if
the Board of Directors has not made such a determination within the three-month
period prior to the Exercise Date, then (A) the Board of Directors shall make a
determination of the Fair Market Value per share of the Common Stock within 15
days of a request by the Registered Holder that it do so, and (B) the exercise
of this Warrant pursuant to this subsection 1(b) shall be delayed until such
determination is made.

     (c)  Each exercise of this Warrant shall be deemed to have been effected
immediately prior to the close of business on the day on which this Warrant
shall have been surrendered to the Company as provided in subsection 1(a) above
(the "Exercise Date"). At such time, the person or persons in whose name or
names any certificates for Warrant Shares shall be issuable upon such exercise
as provided in subsection 1(d) below shall be deemed to have become the holder
or holders of record of the Warrant Shares represented by such certificates.

     (d)  As soon as practicable after the exercise of this Warrant in full or
in part, and in any event within 10 days thereafter, the Company, at its
expense, will cause to be issued in the name of, and delivered to, the
Registered Holder, or as such Holder (upon payment by such Holder of any
applicable transfer taxes) may direct:

          (i)    a certificate or certificates for the number of full Warrant
Shares to which the Registered Holder shall be entitled upon such exercise plus,
in lieu of any fractional share to which the Registered Holder would otherwise
be entitled, cash in an amount determined pursuant to Section 3 hereof; and

          (ii)   in case such exercise is in part only, a new warrant or
warrants (dated the date hereof) of like tenor, calling in the aggregate on the
face or faces thereof for the number of Warrant Shares equal (without giving
effect to any adjustment therein) to the number of such shares called for on the
face of this Warrant minus the sum of (a) the number of such shares purchased by
the Registered Holder upon such exercise plus (b) the number of Warrant


                                      -2-
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Shares (if any) covered by the portion of this Warrant cancelled in payment of
the Purchase Price payable upon such exercise pursuant to subsection 1(b) above.

2.  Adjustments.
    -----------

    (a)  Diluting Issuances.
         ------------------

         (i)   Definitions.  For purposes of this Section 2, the following
definitions shall apply:

               (A)  "Option" shall mean rights, options or warrants to
subscribe for, purchase or otherwise acquire Common Stock or Convertible
Securities.

               (B)  "Original Issue Date" shall mean the date on which this
Warrant was first issued.

               (C)  "Convertible Securities" shall mean any evidences of
indebtedness, shares or other securities directly or indirectly convertible
into or exchangeable for Common Stock, but excluding Options.

               (D)  "Additional Shares of Common Stock" shall mean all shares
of Common Stock issued (or, pursuant to subsection 2(a)(iii) below, deemed to be
issued) by the Company after the Original Issue Date, other than:

               (I)    shares of Common Stock issued or issuable upon conversion
                      or exchange of any Convertible Securities or exercise of
                      any Options outstanding on the Original Issue Date;

               (II)   shares of Common Stock issued or issuable by reason of a
                      dividend, stock split, split-up or other distribution on
                      shares of Common Stock that are covered by subsections
                      2(b) or 2(c) below; or

               (III)  shares of Common Stock (or Options with respect thereto)
                      issued or issuable to employees or directors of, or
                      consultants to, the Company pursuant to a plan or
                      arrangement approved by the Board of Directors of the
                      Company.

         (ii)   No Adjustment of Purchase Price.  No adjustments to the
Purchase Price shall be made unless the consideration per share (determined
pursuant to subsection 2(a)(v)) for an Additional Share of Common Stock issued
or deemed to be issued by the Company is less than the Purchase Price in effect
immediately prior to the issue of such Additional Shares.

         (iii)  Issue of Securities Deemed Issue of Additional Shares of
Common Stock. If the Company at any time or from time to time after the Original
Issue Date shall issue


                                      -3-
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any Options (excluding Options covered by subsection 2(a)(i)(D)(III) above) or
Convertible Securities or shall fix a record date for the determination of
holders of any class of securities entitled to receive any such Options or
Convertible Securities, then the maximum number of shares of Common Stock (as
set forth in the instrument relating thereto without regard to any provision
contained therein for a subsequent adjustment of such number) issuable upon the
exercise of such Options or, in the case of Convertible Securities and Options
therefor, the conversion or exchange of such Convertible Securities, shall be
deemed to be Additional Shares of Common Stock issued as of the time of such
issue or, in case such a record date shall have been fixed, as of the close of
business on such record date, provided that Additional Shares of Common Stock
shall not be deemed to have been issued unless the consideration per share
(determined pursuant to subsection 2(a)(v) hereof) of such Additional Shares of
Common Stock would be less than the Purchase Price in effect on the date of and
immediately prior to such issue, or such record date, as the case may be, and
provided further that in any such case in which Additional Shares of Common
Stock are deemed to be issued:

            (A)  No further adjustment in the Purchase Price shall be made upon
the subsequent issue of Convertible Securities or shares of Common Stock upon
the exercise of such Options or conversion or exchange of such Convertible
Securities;

            (B)  If such Options or Convertible Securities by their terms
provide, with the passage of time or otherwise, for any increase or decrease in
the consideration payable to the Company, then upon the exercise, conversion or
exchange thereof, the Purchase Price computed upon the original issue thereof
(or upon the occurrence of a record date with respect thereto), and any
subsequent adjustments based thereon, shall, upon any such increase or decrease
becoming effective, be recomputed to reflect such increase or decrease insofar
as it affects such Options or the rights of conversion or exchange under such
Convertible Securities;

            (C)  Upon the expiration or termination of any such unexercised
Option, the Purchase Price shall not be readjusted, but the Additional Shares of
Common Stock deemed issued as the result of the original issue of such Option
shall not be deemed issued for the purposes of any subsequent adjustment of the
Purchase Price;

            (D)  In the event of any change in the number of shares of Common
Stock issuable upon the exercise, conversion or exchange of any such Option or
Convertible Security, including, but not limited to, a change resulting from the
anti-dilution provisions thereof, the Purchase Price then in effect shall
forthwith be readjusted to such Purchase Price as would have obtained had the
adjustment which was made upon the issuance of such Option or Convertible
Security not exercised, converted or exchanged prior to such change been made
upon the basis of such change; and

            (E)  No readjustment pursuant to clause (B) or (D) above shall have
the effect of increasing the Purchase Price to an amount which exceeds the lower
of (i) the Purchase Price on the original adjustment date, or (ii) the Purchase
Price that would have resulted from any issuances of Additional Shares of Common
Stock between the original adjustment date and such readjustment date.


                                      -4-
<PAGE>

     In the event the Company, after the Original Issue Date, amends the terms
of any such Options or Convertible Securities (whether such Options or
Convertible Securities were outstanding on the Original Issue Date or were
issued after the Original Issue Date), then such Options or Convertible
Securities, as so amended, shall be deemed to have been issued after the
Original Issue Date and the provisions of this subsection 2(a)(iii) shall apply.

              (iv)  Adjustment of Purchase Price Upon Issuance of Additional
Shares of Common Stock.
                    (A)  In the event the Company shall at any time after the
Original Issue Date issue Additional Shares of Common Stock (including
Additional Shares of Common Stock deemed to be issued pursuant to subsection
2(a)(iii)), without consideration or for a consideration per share less than the
Purchase Price in effect immediately prior to such issue, then and in such
event, such Purchase Price shall be reduced, concurrently with such issue, to a
price (calculated to the nearest cent) determined by multiplying such Purchase
Price by a fraction, (A) the numerator of which shall be (1) the number of
shares of Common Stock outstanding immediately prior to such issue plus (2) the
number of shares of Common Stock which the aggregate consideration received or
to be received by the Company for the total number of Additional Shares of
Common Stock so issued would purchase at such Purchase Price; and (B) the
denominator of which shall be the number of shares of Common Stock outstanding
immediately prior to such issue plus the number of such Additional Shares of
Common Stock so issued; provided that, (i) for the purpose of this subsection
2(a)(iv), all shares of Common Stock issuable upon conversion or exchange of
Convertible Securities outstanding immediately prior to such issue shall be
deemed to be outstanding, and (ii) the number of shares of Common Stock deemed
issuable upon conversion or exchange of such outstanding Convertible Securities
shall not give effect to any adjustments to the conversion or exchange price or
conversion or exchange rate of such Convertible Securities resulting from the
issuance of Additional Shares of Common Stock that is the subject of this
calculation.

                    (B)  Notwithstanding the foregoing, the applicable
Purchase Price shall not be so reduced at such time if the amount of such
reduction would be an amount less than $.01, but any such amount shall be
carried forward and reduction with respect thereto shall be made at the time of
and together with any subsequent reduction which, together with such amount and
any other amount or amounts so carried forward, shall aggregate $.01 or more.

              (v)   Determination of Consideration.  For purposes of this
subsection 2(a), the consideration received by the Company for the issue of any
Additional Shares of Common Stock shall be computed as follows:

                    (A)   Cash and Property:  Such consideration shall:

                    (I)   insofar as it consists of cash, be computed at the
                          aggregate of cash received by the Company, excluding
                          amounts paid or payable for accrued interest;

                    (II)  insofar as it consists of property other than cash, be
                          computed at the fair market value thereof at the time
                          of

                                      -5-
<PAGE>

                       such issue, as determined in good faith by the Board of
                       Directors; and

                (III)  in the event Additional Shares of Common Stock are issued
                       together with other shares or securities or other assets
                       of the Company for consideration which covers both, be
                       the proportion of such consideration so received,
                       computed as provided in clauses (I) and (II) above, as
                       determined in good faith by the Board of Directors.

                (B)    Options and Convertible Securities.  The consideration
per share received by the Company for Additional Shares of Common Stock deemed
to have been issued pursuant to subsection 2(a)(iii), relating to Options and
Convertible Securities, shall be determined by dividing

                       (x)  the total amount, if any, received or receivable by
the Company as consideration for the issue of such Options or Convertible
Securities, plus the minimum aggregate amount of additional consideration (as
set forth in the instruments relating thereto, without regard to any provision
contained therein for a subsequent adjustment of such consideration) payable to
the Company upon the exercise of such Options or the conversion or exchange of
such Convertible Securities, or in the case of Options for Convertible
Securities, the exercise of such Options for Convertible Securities and the
conversion or exchange of such Convertible Securities, by

                       (y)  the maximum number of shares of Common Stock (as
set forth in the instruments relating thereto, without regard to any provision
contained therein for a subsequent adjustment of such number) issuable upon the
exercise of such Options or the conversion or exchange of such Convertible
Securities.

          (vi)  Multiple Closing Dates.  In the event the Company shall issue
on more than one date Additional Shares of Common Stock which are comprised of
shares of the same series or class of Preferred Stock, and such issuance dates
occur within a period of no more than 120 days, then, upon the final such
issuance, the Purchase Price shall be readjusted to give effect to all such
issuances as if they occurred on the date of the final such issuance (and
without giving effect to any adjustments as a result of such prior issuances
within such period).

     (b)  Adjustment for Stock Splits and Combinations.  If the Company shall
at any time or from time to time after the Original Issue Date effect a
subdivision of the outstanding Common Stock, the Purchase Price then in effect
immediately before that subdivision shall be proportionately decreased. If the
Company shall at any time or from time to time after the Original Issue Date
combine the outstanding shares of Common Stock, the Purchase Price then in
effect immediately before the combination shall be proportionately increased.
Any adjustment under this paragraph shall become effective at the close of
business on the date the subdivision or combination becomes effective.

     (c)  Adjustment for Certain Dividends and Distributions.  In the event the
Company at any time, or from time to time after the Original Issue Date shall
make or issue, or


                                      -6-
<PAGE>

fix a record date for the determination of holders of Common Stock entitled to
receive, a dividend or other distribution payable in additional shares of Common
Stock, then and in each such event the Purchase Price then in effect immediately
before such event shall be decreased as of the time of such issuance or, in the
event such a record date shall have been fixed, as of the close of business on
such record date, by multiplying the Purchase Price then in effect by a
fraction:

          (1)  the numerator of which shall be the total number of shares of
     Common Stock issued and outstanding immediately prior to the time of
     such issuance or the close of business on such record date, and

          (2)  the denominator of which shall be the total number of shares
     of Common Stock issued and outstanding immediately prior to the time
     of such issuance or the close of business on such record date plus the
     number of shares of Common Stock issuable in payment of such dividend
     or distribution;

   provided, however, if such record date shall have been fixed and such
dividend is not fully paid or if such distribution is not fully made on the date
fixed therefor, the Purchase Price shall be recomputed accordingly as of the
close of business on such record date and thereafter the Purchase Price shall be
adjusted pursuant to this paragraph as of the time of actual payment of such
dividends or distributions.

     (d)  Adjustment in Number of Warrant Shares.  When any adjustment is
required to be made in the Purchase Price pursuant to subsections 2(a), 2(b) or
2(c), the number of Warrant Shares purchasable upon the exercise of this Warrant
shall be changed to the number determined by dividing (i) an amount equal to the
number of shares issuable upon the exercise of this Warrant immediately prior to
such adjustment, multiplied by the Purchase Price in effect immediately prior to
such adjustment, by (ii) the Purchase Price in effect immediately after such
adjustment.

     (e)  Adjustments for Other Dividends and Distributions.  In the event the
Company at any time or from time to time after the Original Issue Date shall
make or issue, or fix a record date for the determination of holders of Common
Stock entitled to receive, a dividend or other distribution payable in
securities of the Company (other than shares of Common Stock) or in cash or
other property (other than cash out of earnings or earned surplus, determined in
accordance with generally accepted accounting principles), then and in each such
event provision shall be made so that the Registered Holder shall receive upon
exercise hereof, in addition to the number of shares of Common Stock issuable
hereunder, the kind and amount of securities of the Company and/or cash and
other property which the Registered Holder would have been entitled to receive
had this Warrant been exercised into Common Stock on the date of such event and
had the Registered Holder thereafter, during the period from the date of such
event to and including the Exercise Date, retained any such securities
receivable, giving application to all adjustments called for during such period
under this Section 2 with respect to the rights of the Registered Holder.

     (f)  Adjustment for Mergers or Reorganizations, etc. If there shall occur
any reorganization, recapitalization, consolidation or merger involving the
Company in which the

                                      -7-
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Common Stock is converted into or exchanged for securities, cash or other
property (other than a transaction covered by subsections 2(b), 2(c) or 2(e)),
then, following any such reorganization, recapitalization, consolidation or
merger, the Registered Holder shall receive upon exercise hereof the kind and
amount of securities, cash or other property which the Registered Holder would
have been entitled to receive if, immediately prior to such reorganization,
recapitalization, consolidation or merger, the Registered Holder had held the
number of shares of Common Stock subject to this Warrant. In any such case,
appropriate adjustment (as determined in good faith by the Board of Directors of
the Company) shall be made in the application of the provisions set forth herein
with respect to the rights and interests thereafter of the Registered Holder, to
the end that the provisions set forth in this Section 2 (including provisions
with respect to changes in and other adjustments of the Purchase Price) shall
thereafter be applicable, as nearly as reasonably may be, in relation to any
securities, cash or other property thereafter deliverable upon the exercise of
this Warrant.

         (g)  Certificate as to Adjustments.  Upon the occurrence of each
adjustment or readjustment of the Purchase Price pursuant to this Section 2, the
Company at its expense shall promptly compute such adjustment or readjustment in
accordance with the terms hereof and furnish to the Registered Holder a
certificate setting forth such adjustment or readjustment (including the kind
and amount of securities, cash or other property for which this Warrant shall be
exercisable and the Purchase Price) and showing in detail the facts upon which
such adjustment or readjustment is based. The Company shall, upon the written
request at any time of the Registered Holder, furnish or cause to be furnished
to the Registered Holder a certificate setting forth (i) the Purchase Price then
in effect and (ii) the number of shares of Common Stock and the amount, if any,
of other securities, cash or property which then would be received upon the
exercise of this Warrant.

     3.  Fractional Shares.  The Company shall not be required upon the
exercise of this Warrant to issue any fractional shares, but shall make an
adjustment therefor in cash on the basis of the Fair Market Value per share of
Common Stock, as determined pursuant to subsection 1(b) above.

     4.  Requirements for Transfer.

         (a)  This Warrant and the Warrant Shares shall not be sold or
transferred unless either (i) they first shall have been registered under the
Securities Act of 1933, as amended (the "Act"), or (ii) the Company first shall
have been furnished with an opinion of legal counsel, reasonably satisfactory to
the Company, to the effect that such sale or transfer is exempt from the
registration requirements of the Act.

         (b)  Notwithstanding the foregoing, no registration or opinion of
counsel shall be required for (i) a transfer by a Registered Holder which is a
corporation to a wholly owned subsidiary of such corporation, a transfer by a
Registered Holder which is a partnership to a partner of such partnership or a
retired partner of such partnership or to the estate of any such partner or
retired partner, or a transfer by a Registered Holder which is a limited
liability company to a member of such limited liability company or a retired
member or to the estate of any such member or retired member, provided that the
transferee in each case agrees in writing


                                      -8-
<PAGE>

to be subject to the terms of this Section 4, or (ii) a transfer made in
accordance with Rule 144 under the Act.

          (c)  Each certificate representing Warrant Shares shall bear a legend
substantially in the following form:

          "The securities represented by this certificate have not been
          registered under the Securities Act of 1933, as amended, and may not
          be offered, sold or otherwise transferred, pledged or hypothecated
          unless and until such securities are registered under such Act or an
          opinion of counsel satisfactory to the Company is obtained to the
          effect that such registration is not required."

      The foregoing legend shall be removed from the certificates representing
any Warrant Shares, at the request of the holder thereof, at such time as they
become eligible for resale pursuant to Rule 144(k) under the Act.

      5.  No Impairment.  The Company will not, by amendment of its charter or
through any reorganization, transfer of assets, consolidation, merger,
dissolution, issue or sale of securities or any other voluntary action, avoid or
seek to avoid the observance or performance of any of the terms of this Warrant,
but will at all times in good faith assist in the carrying out of all such terms
and in the taking of all such action as may be necessary or appropriate in order
to protect the rights of the holder of this Warrant against impairment.

      6.  Notices of Record Date, etc.  In the event:

          (a)  the Company shall take a record of the holders of its Common
Stock (or other stock or securities at the time deliverable upon the exercise of
this Warrant) for the purpose of entitling or enabling them to receive any
dividend or other distribution, or to receive any right to subscribe for or
purchase any shares of stock of any class or any other securities, or to receive
any other right; or

          (b)  of any capital reorganization of the Company, any
reclassification of the Common Stock of the Company, any consolidation or merger
of the Company with or into another corporation (other than a consolidation or
merger in which the Company is the surviving entity and its Common Stock is not
converted into or exchanged for any other securities or property), or any
transfer of all or substantially all of the assets of the Company; or

          (c)  of the voluntary or involuntary dissolution, liquidation or
winding-up of the Company,

then, and in each such case, the Company will mail or cause to be mailed to the
Registered Holder a notice specifying, as the case may be, (i) the record date
for such dividend, distribution or right, and the amount and character of such
dividend, distribution or right, or (ii) the effective date on which such
reorganization, reclassification, consolidation, merger, transfer, dissolution,
liquidation or winding-up is to take place, and the time, if any is to be fixed,
as of which the holders of record of Common Stock (or such other stock or
securities at the time deliverable upon the exercise of this Warrant) shall be
entitled to exchange their shares of Common Stock

                                      -9-
<PAGE>

(or such other stock or securities) for securities or other property deliverable
upon such reorganization, reclassification, consolidation, merger, transfer,
dissolution, liquidation or winding-up. Such notice shall be mailed at least ten
days prior to the record date or effective date for the event specified in such
notice.

     7.   Reservation of Stock.  The Company will at all times reserve and keep
available, solely for issuance and delivery upon the exercise of this Warrant,
such number of Warrant Shares and other securities, cash and/or property, as
from time to time shall be issuable upon the exercise of this Warrant.

     8.   Exchange of Warrants.  Upon the surrender by the Registered Holder,
properly endorsed, to the Company at the principal office of the Company, the
Company will, subject to the provisions of Section 4 hereof, issue and deliver
to or upon the order of such Holder, at the Company's expense, a new Warrant or
Warrants of like tenor, in the name of the Registered Holder or as the
Registered Holder (upon payment by the Registered Holder of any applicable
transfer taxes) may direct, calling in the aggregate on the face or faces
thereof for the number of shares of Common Stock (or other securities, cash
and/or property) then issuable upon exercise of this Warrant.

     9.   Replacement of Warrants.  Upon receipt of evidence reasonably
satisfactory to the Company of the loss, theft, destruction or mutilation of
this Warrant and (in the case of loss, theft or destruction) upon delivery of an
indemnity agreement (with surety if reasonably required) in an amount reasonably
satisfactory to the Company, or (in the case of mutilation) upon surrender and
cancellation of this Warrant, the Company will issue, in lieu thereof, a new
Warrant of like tenor.

     10.  Transfers, etc.

          (a)  The Company will maintain a register containing the name and
address of the Registered Holder of this Warrant. The Registered Holder may
change its or his address as shown on the warrant register by written notice to
the Company requesting such change.

          (b)  Subject to the provisions of Section 4 hereof, this Warrant and
all rights hereunder are transferable, in whole or in part, upon surrender of
this Warrant with a properly executed assignment (in the form of Exhibit II
hereto) at the principal office of the Company.

          (c)  Until any transfer of this Warrant is made in the warrant
register, the Company may treat the Registered Holder as the absolute owner
hereof for all purposes; provided, however, that if and when this Warrant is
properly assigned in blank, the Company may (but shall not be obligated to)
treat the bearer hereof as the absolute owner hereof for all purposes,
notwithstanding any notice to the contrary.

     11.  Mailing of Notices, etc.  All notices and other communications from
the Company to the Registered Holder shall be mailed by first-class certified or
registered mail, postage prepaid, to the address last furnished to the Company
in writing by the Registered Holder. All notices and other communications from
the Registered Holder or in connection herewith to the Company shall be mailed
by first-class certified or registered mail, postage prepaid, to the Company at
its principal office set forth below. If the Company should at any time change
the

                                      -10-
<PAGE>

location of its principal office to a place other than as set forth below,
it shall give prompt written notice to the Registered Holder and thereafter all
references in this Warrant to the location of its principal office at the
particular time shall be as so specified in such notice.

                 The Company:

                 NMT Medical, Inc.
                 27 Wormwood Street
                 Boston, MA  02110
                 Facsimile Number: (617) 737-1267
                 Attention: Chief Financial Officer

      12.  No Rights as Stockholder.  Until the exercise of this Warrant, the
Registered Holder shall not have or exercise any rights by virtue hereof as a
stockholder of the Company.  Notwithstanding the foregoing, in the event (i) the
Company effects a split of the Common Stock by means of a stock dividend and the
Purchase Price of and the number of Warrant Shares are adjusted as of the date
of the distribution of the dividend (rather than as of the record date for such
dividend), and (ii) the Registered Holder exercises this Warrant between the
record date and the distribution date for such stock dividend, the Registered
Holder shall be entitled to receive, on the distribution date, the stock
dividend with respect to the shares of Common Stock acquired upon such exercise,
notwithstanding the fact that such shares were not outstanding as of the close
of business on the record date for such stock dividend.

      13.  Change or Waiver.  Any term of this Warrant may be changed or waived
only by an instrument in writing signed by the party against which enforcement
of the change or waiver is sought.

      14.  Section Headings.  The section headings in this Warrant are for the
convenience of the parties and in no way alter, modify, amend, limit or restrict
the contractual obligations of the parties.

      15.  Governing Law.  This Warrant will be governed by and construed in
accordance with the internal laws of the Commonwealth of Massachusetts (without
reference to the conflicts of law provisions thereof).

      EXECUTED as of the Date of Issuance indicated above.

                              NMT Medical, Inc.


                              By: /s/ William J. Knight
                                 ----------------------
[Corporate Seal]              Title: Vice President and
                                     ------------------
                                     Chief Financial Officer
                                     -----------------------

ATTEST:
/s/ Ian J. Platt
-------------------------

                                      -11-
<PAGE>

                                                                       EXHIBIT I
                                                                       ---------

                                 PURCHASE FORM
                                 -------------

To:_________________                                       Dated:_______________

     The undersigned, pursuant to the provisions set forth in the attached
Warrant (No. BBH-1), hereby irrevocably elects to purchase (check applicable
box):

     [ ]  _____ shares of the Common Stock covered by such Warrant; or

     [ ]  the maximum number of shares of Common Stock covered by such
          Warrant pursuant to the cashless exercise procedure set forth in
          Section 1(b).

     The undersigned herewith makes payment of the full purchase price for such
shares at the price per share provided for in such Warrant, which is $________.
Such payment takes the form of (check applicable box or boxes):

     [ ]  $______ in lawful money of the United States; and/or

     [ ]  the cancellation of such portion of the attached Warrant as
          is exercisable for a total of _____ Warrant Shares (using a Fair
          Market Value of $_____ per share for purposes of this calculation);
          and/or

     [ ]  the cancellation of such number of Warrant Shares as is
          necessary, in accordance with the formula set forth in Section 1(b),
          to exercise this Warrant with respect to the maximum number of Warrant
          Shares purchasable pursuant to the cashless exercise procedure set
          forth in Section 1(b).

                              Signature:_____________________

                              Address:  _____________________

                                        _____________________



                                      -12-
<PAGE>

                                                                      EXHIBIT II
                                                                      ----------

                                ASSIGNMENT FORM
                                ---------------

     FOR VALUE RECEIVED, ________________________________________ hereby sells,
assigns and transfers all of the rights of the undersigned under the attached
Warrant (No. BBH-1) with respect to the number of shares of Common Stock covered
thereby set forth below, unto:

Name of Assignee        Address                No. of Shares
----------------        -------                -------------





Dated:_____________________         Signature:________________________________
                                              Signature Guaranteed:

By: _______________________
the signature should be guaranteed
by an eligible guarantor institution
(banks, stockbrokers, savings and loan
associations and credit unions with
membership in an approved signature
guarantee medallion program) pursuant
to Rule 17Ad-15 under the Securities
Exchange Act of 1934.




                                      -13-