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=========================================================================







                          GUARANTEE AGREEMENT


                                Between


                         ALLEGIANT BANCORP, INC.
                             (as Guarantor)


                                  and


                         BANKERS TRUST COMPANY
                        (as Guarantee Trustee)


                              dated as of


                             August 2, 1999




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                 ALLEGIANT CAPITAL TRUST I

      Certain Sections of this Guarantee Agreement relating to
                  Sections 310 through 318 of the
                    Trust Indenture Act of 1939:


Trust Indenture                                   Guarantee Agreement
Act Section                                             Section
---------------                                   -------------------

Section 310    (a)(1)                                          4.1(a)
               (a)(2)                                          4.1(a)
               (a)(3)                                  Not Applicable
               (a)(4)                                  Not Applicable
               (b)                                        2.8, 4.1(c)
Section 311    (a)                                     Not Applicable
               (b)                                     Not Applicable
Section 312    (a)                                             2.2(a)
               (b)                                             2.2(b)
               (c)                                     Not Applicable
Section 313    (a)                                                2.3
               (a)(4)                                             2.3
               (b)                                                2.3
               (c)                                                2.3
               (d)                                                2.3
Section 314    (a)                                                2.4
               (b)                                                2.4
               (c)(1)                                             2.5
               (c)(2)                                             2.5
               (c)(3)                                             2.5
               (e)                                      1.1, 2.5, 3.2
Section 315    (a)                                             3.1(d)
               (b)                                                2.7
               (c)                                             3.1(c)
               (d)                                             3.1(d)
               (e)                                     Not Applicable
Section 316    (a)                                      1.1, 2.6, 5.4
               (a)(1)(A)                                          5.4
               (a)(1)(B)                                          5.4
               (a)(2)                                  Not Applicable
               (b)                                                5.3
               (c)                                     Not Applicable
Section 317    (a)(1)                                  Not Applicable
               (a)(2)                                  Not Applicable
               (b)                                     Not Applicable
Section 318    (a)                                                2.1


Note: This reconciliation and tie shall not, for any purpose, be deemed
      to be a part of the Guarantee Agreement.


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                            TABLE OF CONTENTS


ARTICLE I      DEFINITIONS                                               1
Section 1.1.   Definitions                                               1

ARTICLE II     TRUST INDENTURE ACT                                       5
Section 2.1.   Trust Indenture Act; Application                          5
Section 2.2.   List of Holders                                           5
Section 2.3.   Annual Reports by the Guarantee Trustee                   5
Section 2.4.   Periodic Reports to the Guarantee Trustee                 5
Section 2.5.   Evidence of Compliance with Conditions Precedent          6
Section 2.6.   Events of Default; Waiver                                 6
Section 2.7.   Event of Default; Notice                                  6
Section 2.9.   Conflicting Interests                                     6

ARTICLE III    POWERS, DUTIES AND RIGHTS OF THE GUARANTEE TRUSTEE        6
Section 3.1.   Powers and Duties of the Guarantee Trustee                6
Section 3.2.   Certain Rights of Guarantee Trustee                       8
Section 3.3.   Indemnity                                                 9
Section 3.4.   Expenses                                                 10

ARTICLE IV     GUARANTEE TRUSTEE                                        10
Section 4.1.   Guarantee Trustee; Eligibility                           10
Section 4.2.   Appointment, Removal and Resignation of the Guarantee
               Trustee                                                  10

ARTICLE V      GUARANTEE                                                11
Section 5.1.   Guarantee                                                11
Section 5.2.   Waiver of Notice and Demand                              11
Section 5.3.   Obligations Not Affected                                 11
Section 5.4.   Rights of Holders                                        12
Section 5.5.   Guarantee of Payment                                     13
Section 5.6.   Subrogation                                              13
Section 5.7.   Independent Obligations                                  13

ARTICLE VI     COVENANTS AND SUBORDINATION                              13
Section 6.1.   Subordination                                            13
Section 6.2.   Pari Passu Guarantees                                    14

ARTICLE VII    TERMINATION                                              14
Section 7.1.   Termination                                              14

ARTICLE VIII   MISCELLANEOUS                                            14
Section 8.1.   Successors and Assigns                                   14
Section 8.2.   Amendments                                               14
Section 8.3.   Notices                                                  14
Section 8.4.   Benefit                                                  16
Section 8.5.   Interpretation                                           16
Section 8.6.   Governing Law                                            16
Section 8.7.   Counterparts                                             16

                                - i -



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                        GUARANTEE AGREEMENT


     THIS GUARANTEE AGREEMENT, dated as of August 2, 1999 (this "Guarantee
Agreement"), is executed and delivered by ALLEGIANT BANCORP, INC., a
Missouri corporation (the "Guarantor"), having its principal office at
2122 Kratky Road, St. Louis, Missouri 63114 and BANKERS TRUST COMPANY, a
New York banking corporation, having its principal office at Four Albany
Street, Fourth Floor, New York, New York 10006, as trustee, for the
benefit of the Holders (as defined herein) from time to time of the
Preferred Securities (as defined herein) of Allegiant Capital Trust I, a
Delaware statutory business trust (the "Issuer Trust").

                              RECITALS

     WHEREAS, pursuant to an Amended and Restated Trust Agreement (the
"Trust Agreement"), dated as of August 2, 1999, among Allegiant Bancorp,
Inc., as Depositor, Bankers Trust Company, as Property Trustee (the
"Property Trustee"), Bankers Trust (Delaware), as Delaware Trustee (the
"Delaware Trustee") (collectively, the "Issuer Trustees"), the
Administrators named therein and the Holders from time to time of
preferred undivided beneficial ownership interests in the assets of
the Issuer Trust, the Issuer Trust is issuing up to $17,250,000 aggregate
Liquidation Amount (as defined herein) of its 9.875% Preferred Securities,
Liquidation Amount $10 per preferred security (the "Preferred
Securities"), (including up to 225,000 preferred securities that may be
issued to cover over-allotments), representing preferred undivided
beneficial ownership interests in the assets of the Issuer Trust and
having the terms set forth in the Trust Agreement;

     WHEREAS, the Preferred Securities will be issued by the Issuer
Trust and the proceeds thereof, together with the proceeds from the
issuance of the Issuer Trust's Common Securities (as defined herein),
will be used to purchase the Junior Subordinated Debentures due August 2,
2029 (as defined in the Trust Agreement) (the "Junior Subordinated
Debentures") of the Guarantor which will be deposited with Bankers Trust
Company, as Property Trustee under the Trust Agreement, as trust assets;
and

     WHEREAS, as incentive for the Holders to purchase the Preferred
Securities, the Guarantor desires irrevocably and unconditionally to
agree to the extent set forth herein, to pay to the Holders of the
Preferred Securities the Guarantee Payments (as defined herein) and to
make certain other payments on the terms and conditions set forth
herein.

     NOW, THEREFORE, in consideration of the purchase of the Preferred
Securities by each Holder, which purchase the Guarantor hereby
acknowledges shall benefit the Guarantor, and intending to be legally
bound hereby, the Guarantor executes and delivers this Guarantee
Agreement for the benefit of the Holders from time to time of the
Preferred Securities.

                             ARTICLE I

                            DEFINITIONS

     Section 1.1.    Definitions

     As used in this Guarantee Agreement, the terms set forth below
shall, unless the context otherwise requires, have the following
meanings. Capitalized terms used but not otherwise defined herein shall
have the meanings assigned to such terms in the Trust Agreement as in
effect on the date hereof.

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     "Act" shall have the meaning specified in the Trust Agreement.

     "Additional Amount" shall have the meaning specified in the Trust
Agreement.

     "Affiliate" of any specified Person means any other Person
directly or indirectly controlling or controlled by or under direct or
indirect common control with such specified Person. For the purposes of
this definition, "control," when used with respect to any specified
Person, means the power to direct the management and policies of such
Person, directly or indirectly, whether through the ownership of voting
securities, by contract or otherwise; and the terms "controlling" and
"controlled" have meanings correlative to the foregoing.

     "Cause" means any one of the following reasons:

          (a)   the Guarantee Trustee is incapable of acting as
     Guarantee Trustee;

          (b)   the Guarantee Trustee is adjudged a bankrupt or
     insolvent, or a receiver of the Guarantee Trustee or of its
     property is appointed; or

          (c)   any public officer takes charge or control of the
     Guarantee Trustee or of its property or affairs for the purposes
     of rehabilitation, conservation or liquidation.

     "Common Securities" means the securities representing common
undivided beneficial interests in the assets of the Issuer Trust.

     "Delaware Trustee" shall have the meaning specified in the first
recital of this Guarantee Agreement.

     "Distributions" means preferential cumulative cash distributions
accumulating from August 2, 1999 and payable quarterly in arrears March
31, June 30, September 30 and December 31 of each year, commencing
August 2, 1999 at the annual rate of 9.875% of the Liquidation Amount.

     "Event of Default" means (a) a default by the Guarantor in any of
its payment obligations under this Guarantee Agreement, or (b) a default
by the Guarantor in any other obligation hereunder that remains
unremedied for 30 days.

     "Extended Interest Payment Period" shall have the meaning
specified in the Indenture.

     "Guarantee Agreement" means this Guarantee Agreement, as modified,
amended or supplemented from time to time.

     "Guarantee Payments" means the following payments or
distributions, without duplication, with respect to the Preferred
Securities, to the extent not paid or made by or on behalf of the Issuer
Trust: (a) any accumulated and unpaid Distributions required to be paid
on the Preferred Securities, to the extent the Issuer Trust shall have
funds available therefor at such time; (b) the Redemption Price, with
respect to the Preferred Securities called for redemption by the Issuer
Trust to the extent that the Issuer Trust shall have funds available
therefor at such time; and (c) upon a voluntary or involuntary
termination, winding-up or liquidation of the Issuer Trust, unless the
Junior Subordinated Debentures are distributed to the Holders, the
lesser of (in either case, the "Liquidation Distribution") (i) the
aggregate of

                               - 2 -

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the Liquidation Amount and all accumulated and unpaid Distributions to
the date of payment to the extent the Issuer Trust shall have funds
available to make such payment at such time and (ii) the amount of
assets of the Issuer Trust remaining available for distribution to
Holders in liquidation of the Issuer Trust.

     "Guarantee Trustee" means Bankers Trust Company, until a Successor
Guarantee Trustee has been appointed and has accepted such appointment
pursuant to the terms of this Guarantee Agreement and thereafter means
each such Successor Guarantee Trustee.

     "Guarantor" shall have the meaning specified in the preamble of
this Guarantee Agreement.

     "Holder" means any holder, as registered on the books and records
of the Issuer Trust, of any Preferred Securities; provided, however,
that, in determining whether the holders of the requisite percentage of
Preferred Securities have given any request, notice, consent or waiver
hereunder, "Holder" shall not include the Guarantor, the Guarantee
Trustee or any Affiliate of the Guarantor or the Guarantee Trustee.

     "Indenture" means the Junior Subordinated Indenture dated as of
August 2, 1999, between Allegiant Bancorp, Inc. and Bankers Trust
Company, as trustee, as may be modified, amended or supplemented from
time to time.

     "Issuer Trust" shall have the meaning specified in the preamble of
this Guarantee Agreement.

     "Issuer Trustees" shall have the meaning specified in the first
recital of this Guarantee Agreement.

     "Junior Subordinated Debentures" shall have the meaning specified
in the first recital of this Guarantee Agreement.

     "Like Amount" means (a) with respect to a redemption of Preferred
Securities, Preferred Securities having a Liquidation Amount equal to
the principal amount of Junior Subordinated Debentures to be
contemporaneously redeemed in accordance with the Indenture, the
proceeds of which will be used to pay the Redemption Price of such
Preferred Securities, (b) with respect to a distribution of Junior
Subordinated Debentures to Holders of Preferred Securities in connection
with a termination, winding up or liquidation of the Issuer Trust,
Junior Subordinated Debentures having a principal amount equal to the
Liquidation Amount of the Preferred Securities of the Holder to whom
such Junior Subordinated Debentures are distributed, and (c) with
respect to any distribution of an Additional Amount to Holders of
Preferred Securities, Junior Subordinated Debentures having a principal
amount equal to the Liquidation Amount of the Preferred Securities in
respect of which such distribution is made.

     "Liquidation Amount" means the stated amount of $10 per Preferred
Security.

     "Liquidation Distribution" shall have the meaning specified in the
definition of Guarantee Payments.

     "Majority in Liquidation Amount of the Preferred Securities"
means, except as provided by the Trust Indenture Act, Preferred
Securities representing more than 50% of the aggregate Liquidation
Amount of all then outstanding Preferred Securities issued by the Issuer
Trust.

                               - 3 -

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     "Officers' Certificate" means, with respect to any Person, a
certificate signed by the Chairman of the Board, Chief Executive Officer,
President or a Vice President, and by the Chief Financial Officer,
Treasurer, an Associate Treasurer, an Assistant Treasurer, the Secretary
or an Assistant Secretary of such Person, and delivered to the Guarantee
Trustee. Any Officers' Certificate delivered with respect to compliance
with a condition or covenant provided for in this Guarantee Agreement
shall include:

          (a)   a statement by each officer signing the Officers'
     Certificate that such officer has read the covenant or condition
     and the definitions relating thereto;

          (b)   a brief statement of the nature and scope of the
     examination or investigation undertaken by such officer in
     rendering the Officers' Certificate;

          (c)   a statement that such officer has made such
          examination or investigation as, in such officer's opinion, is
     necessary to enable such officer to express an informed opinion as
     to whether or not such covenant or condition has been complied
     with; and

          (d)   a statement as to whether, in the opinion of such
     officer, such condition or covenant has been complied with.

     "Person" means a legal person, including any individual,
corporation, estate, partnership, joint venture, association, joint
stock company, limited liability company, trust, unincorporated
association, or government or any agency or political subdivision
thereof, or any other entity of whatever nature.

     "Preferred Securities" shall have the meaning specified in the
first recital of this Guarantee Agreement.

     "Property Trustee" shall have the meaning specified in the first
recital of this Guarantee Agreement.

     "Redemption Date" means, with respect to any Preferred Security to
be redeemed, the date fixed for such redemption by or pursuant to the
Trust Agreement; provided that each Junior Subordinated Debenture
Redemption Date (as such term is defined in the Indenture) and the
stated maturity of the Junior Subordinated Debentures shall be a
Redemption Date for a Like Amount of Preferred Securities.

     "Redemption Price" shall have the meaning specified in the Trust
Agreement.

     "Responsible Officer" means, when used with respect to the
Guarantee Trustee, any officer assigned to the Corporate Trust Office of
the Guarantee Trustee, including any managing director, director, vice
president, assistant vice president, associate or any other officer of
the Guarantee Trustee customarily performing functions similar to those
performed by any of the above designated officers and having direct
responsibility for the administration of this Guarantee Agreement, and
also, with respect to a particular matter, any other officer to whom
such matter is referred because of such officer's knowledge of and
familiarity with the particular subject.

     "Senior Indebtedness" shall have the meaning specified in the
Indenture.

     "Successor Guarantee Trustee" means a successor Guarantee Trustee
possessing the qualifications to act as Guarantee Trustee under Section
4.1.

                               - 4 -


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     "Trust Agreement" shall have the meaning specified in the Recitals
to this Guarantee Agreement.

     "Trust Indenture Act" means the Trust Indenture Act of 1939, as
amended by the Trust Indenture Reform Act of 1990, or any successor
statute, in each case as amended from time to time.

                             ARTICLE II

                        TRUST INDENTURE ACT

     Section 2.1.    Trust Indenture Act; Application.

     If any provision hereof limits, qualifies or conflicts with a
provision of the Trust Indenture Act that is required to be a part of
and govern this Guarantee Agreement, the provision of the Trust
Indenture Act shall control. If any provision of this Guarantee
Agreement modifies or excludes any provision of the Trust Indenture Act
that may be so modified or excluded, the latter provision shall be
deemed to apply to this Guarantee Agreement as so modified or excluded,
as the case may be.

     Section 2.2.    List of Holders.

          (a)   The Guarantor will furnish or cause to be furnished
     to the Guarantee Trustee:

                (i) quarterly, not more than 15 days after March
          15, June 15, September 15 and December 15 in each year, a
          list, in such form as the Guarantee Trustee may reasonably
          require, of the names and addresses of the Holders as of
          such date; and

                (ii) at such other times as the Guarantee Trustee
          may request in writing, within 30 days after the receipt by
          the Guarantor of any such request, a list of similar form
          and content as of a date not more than 15 days prior to the
          time such list is furnished.

          (b)   The Guarantee Trustee shall comply with the
     requirements of Section 312(b) of the Trust Indenture Act.

     Section 2.3.    Annual Reports by the Guarantee Trustee.

     Within 60 days of January 31 of each year commencing January 31,
2000, the Guarantee Trustee shall provide to the Holders such reports,
if any, as are required by Section 313 of the Trust Indenture Act in the
form and in the manner provided by Section 313 of the Trust Indenture
Act.  The Guarantee Trustee also shall comply with the requirements of
Section 313(d) of the Trust Indenture Act.

     Section 2.4.    Periodic Reports to the Guarantee Trustee.

     The Guarantor shall provide to the Guarantee Trustee and the
Holders such documents, reports and information, if any, as required by
Section 314 of the Trust Indenture Act and the compliance certificate
required by Section 314 of the Trust Indenture Act, in the form, in the
manner and at the times required by Section 314 of the Trust Indenture
Act.

                               - 5 -


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     Section 2.5.    Evidence of Compliance with Conditions Precedent.

     The Guarantor shall provide to the Guarantee Trustee such evidence
of compliance with such conditions precedent, if any, provided for in
this Guarantee Agreement that relate to any of the matters set forth in
Section 314(c) of the Trust Indenture Act. Any certificate or opinion
required to be given by an officer pursuant to Section 314(c)(1) may be
given in the form of an Officers' Certificate.

     Section 2.6.    Events of Default; Waiver.

     The Holders of a Majority in Liquidation Amount of the Preferred
Securities may, by vote, on behalf of the Holders, waive any past Event
of Default and its consequences. Upon such waiver, any such Event of
Default shall cease to exist, and any Event of Default arising therefrom
shall be deemed to have been cured, for every purpose of this Guarantee
Agreement, but no such waiver shall extend to any subsequent or other
default or Event of Default or impair any right consequent therefrom.

     Section 2.7.    Event of Default; Notice.

          (a)   The Guarantee Trustee shall, within 90 days after
     the occurrence of an Event of Default, transmit by mail, first
     class postage prepaid, to the Holders, notices of all Events of
     Default known to the Guarantee Trustee, unless such Events of
     Default have been cured before the giving of such notice; provided
     that, except in the case of a default in the payment of a
     Guarantee Payment, the Guarantee Trustee shall be protected in
     withholding such notice if and so long as the Board of Directors,
     the executive committee or a trust committee of directors and/or
     Responsible Officers of the Guarantee Trustee in good faith
     determines that the withholding of such notice is in the interests
     of the Holders.

          (b)   The Guarantee Trustee shall not be deemed to have
     knowledge of any Event of Default unless (i) a Responsible Officer
     charged with the administration of this Guarantee Agreement shall
     have received written notice of such Event of Default, or (ii) a
     Responsible Officer of the Guarantee Trustee charged with
     administration of the Trust Agreement shall have obtained actual
     knowledge thereof.

     Section 2.8.    Conflicting Interests.

     The Trust Agreement shall be deemed to be specifically described
in this Guarantee Agreement for the purposes of clause (i) of the first
proviso contained in Section 310(b) of the Trust Indenture Act.

                            ARTICLE III

         POWERS, DUTIES AND RIGHTS OF THE GUARANTEE TRUSTEE

     Section 3.1.    Powers and Duties of the Guarantee Trustee.

          (a)   This Guarantee Agreement shall be held by the
     Guarantee Trustee for the benefit of the Holders, and the
     Guarantee Trustee shall not transfer this Guarantee Agreement to
     any Person except to a Holder exercising his or her rights
     pursuant to Section 5.4(d) or to a Successor Guarantee Trustee on
     acceptance by such Successor Guarantee Trustee of its appointment
     to act as Successor Guarantee Trustee hereunder. The right, title
     and interest of the Guarantee Trustee, as such, hereunder shall
     automatically vest in any Successor Guarantee Trustee, upon
     acceptance

                               - 6 -


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     by such Successor Guarantee Trustee of its appointment hereunder,
     and such vesting and cessation of title shall be effective whether
     or not conveyancing documents have been executed and delivered
     pursuant to the appointment of such Successor Guarantee Trustee.

          (b)   If an Event of Default has occurred and is
     continuing, the Guarantee Trustee shall enforce this Guarantee
     Agreement for the benefit of the Holders.

          (c)   The Guarantee Trustee, before the occurrence of any
     Event of Default and after the curing of all Events of Default
     that may have occurred, shall be obligated to perform only such
     duties as are specifically set forth in this Guarantee Agreement
     (including pursuant to Section 2.1), and no implied covenants
     shall be read into this Guarantee Agreement against the Guarantee
     Trustee. If an Event of Default has occurred (that has not been
     cured or waived pursuant to Section 2.6), the Guarantee Trustee
     shall exercise such of the rights and powers vested in it by this
     Guarantee Agreement, and use the same degree of care and skill in
     its exercise thereof, as a prudent person would exercise or use
     under the circumstances in the conduct of his or her own affairs.

          (d)   No provision of this Guarantee Agreement shall be
     construed to relieve the Guarantee Trustee from liability for its
     own negligent action, its own negligent failure to act or its own
     bad faith or willful misconduct, except that:

                (i) prior to the occurrence of any Event of
          Default and after the curing or waiving of all such Events
          of Default that may have occurred:

                     (A) the duties and obligations of the
                Guarantee Trustee shall be determined solely by the
                express provisions of this Guarantee Agreement
                (including pursuant to Section 2.1), and the
                Guarantee Trustee shall not be liable except for the
                performance of such duties and obligations as are
                specifically set forth in this Guarantee Agreement
                (including pursuant to Section 2.1); and

                     (B) in the absence of bad faith on the
                part of the Guarantee Trustee, the Guarantee Trustee
                may conclusively rely, as to the truth of the
                statements and the correctness of the opinions
                expressed therein, upon any certificates or opinions
                furnished to the Guarantee Trustee and conforming to
                the requirements of this Guarantee Agreement; but in
                the case of any such certificates or opinions that
                by any provision hereof or of the Trust Indenture
                Act are specifically required to be furnished to the
                Guarantee Trustee, the Guarantee Trustee shall be
                under a duty to examine the same to determine
                whether or not they conform to the requirements of
                this Guarantee Agreement;

                (ii) the Guarantee Trustee shall not be liable for
          any error of judgment made in good faith by a Responsible
          Officer of the Guarantee Trustee, unless it shall be proved
          that the Guarantee Trustee was negligent in ascertaining the
          pertinent facts upon which such judgment was made;

                (iii) the Guarantee Trustee shall not be liable
          with respect to any action taken or omitted to be taken by
          it in good faith in accordance with the direction of the
          Holders of not less than a Majority in Liquidation Amount of
          the Preferred Securities relating to the time, method and
          place of conducting any proceeding for any remedy available
          to the

                               - 7 -


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          Guarantee Trustee, or exercising any trust or power
          conferred upon the Guarantee Trustee under this Guarantee
          Agreement; and

                (iv) no provision of this Guarantee Agreement
          shall require the Guarantee Trustee to expend or risk its
          own funds or otherwise incur personal financial liability in
          the performance of any of its duties or in the exercise of
          any of its rights or powers if the Guarantee Trustee shall
          have reasonable grounds for believing that the repayment of
          such funds or liability is not assured to it under the terms
          of this Guarantee Agreement or adequate indemnity against
          such risk or liability is not reasonably assured to it.

     Section 3.2.    Certain Rights of Guarantee Trustee.

          (a)   Subject to the provisions of Section 3.1:

                (i) the Guarantee Trustee may conclusively rely
          and shall be fully protected in acting or refraining from
          acting upon any resolution, certificate, statement,
          instrument, opinion, report, notice, request, direction,
          consent, order, bond, debenture, note, other evidence of
          indebtedness or other paper or document reasonably believed
          by it to be genuine and to have been signed, sent or
          presented by the proper party or parties;

                (ii) any direction or act of the Guarantor
          contemplated by this Guarantee Agreement shall be
          sufficiently evidenced by an Officers' Certificate unless
          otherwise prescribed herein;

                (iii) whenever, in the administration of this
          Guarantee Agreement, the Guarantee Trustee shall deem it
          desirable that a matter be proved or established before
          taking, suffering or omitting to take any action hereunder,
          the Guarantee Trustee (unless other evidence is herein
          specifically prescribed) may, in the absence of bad faith on
          its part, request and conclusively rely upon an Officers'
          Certificate which, upon receipt of such request from the
          Guarantee Trustee, shall be promptly delivered by the
          Guarantor;

                (iv) the Guarantee Trustee may consult with legal
          counsel, and the written advice or opinion of such legal
          counsel with respect to legal matters shall be full and
          complete authorization and protection in respect of any
          action taken, suffered or omitted to be taken by it
          hereunder in good faith and in accordance with such advice
          or opinion. Such legal counsel may be legal counsel to the
          Guarantor or any of its Affiliates and may be one of its
          employees. The Guarantee Trustee shall have the right at any
          time to seek instructions concerning the administration of
          this Guarantee Agreement from any court of competent
          jurisdiction;

                (v) the Guarantee Trustee shall be under no
          obligation to exercise any of the rights or powers vested
          in it by this Guarantee Agreement at the request or
          direction of any Holder, unless such Holder shall have
          provided to the Guarantee Trustee such security and
          indemnity as would satisfy a reasonable person in the
          position of the Guarantee Trustee, against the costs,
          expenses (including attorneys' fees and expenses) and
          liabilities that might be incurred by it in complying with
          such request or direction, including such reasonable
          advances as may be requested by the Guarantee Trustee;
          provided, however, that nothing herein shall relieve the
          Guarantee Trustee of its obligations upon the occurrence of
          an Event of Default that has not been cured or waived

                               - 8 -


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          to exercise the rights and powers vested in the Guarantee
          Trustee by this Guarantee, and to use the same degree of
          care and skill in exercising such rights and powers as a
          reasonably prudent person would use under the circumstances
          in the conduct of his own affairs;

                (vi) the Guarantee Trustee shall not be bound to
          make any investigation into the facts or matters stated in
          any resolution, certificate, statement, instrument, opinion,
          report, notice, request, direction, consent, order, bond,
          debenture, note, other evidence of indebtedness or other
          paper or document, but the Guarantee Trustee, in its
          discretion, may make such further inquiry or investigation
          into such facts or matters as it may see fit;

                (vii) the Guarantee Trustee may execute any of the
          trusts or powers hereunder or perform any duties hereunder
          either directly or by or through its agents or attorneys,
          and the Guarantee Trustee shall not be responsible for any
          negligence or willful misconduct on the part of any such
          agent or attorney appointed with due care by it hereunder.
          Nothing herein shall be construed as limiting or restricting
          the right of the Guarantor to bring any action directly
          against any agent or attorney appointed by the Guarantee
          Trustee for any negligence or willful misconduct on the part
          of such agent or attorney; and

                (viii) whenever in the administration of this
          Guarantee Agreement the Guarantee Trustee shall deem it
          desirable to receive instructions with respect to enforcing
          any remedy or right or taking any other action hereunder,
          the Guarantee Trustee:

                     (A) may request instructions from the
                Holders;

                     (B) may refrain from enforcing such remedy
                or right or taking such other action until such
                instructions are received; and

                     (C) shall be fully protected in acting in
                accordance with such instructions.

          (b)   No provision of this Guarantee Agreement shall be
     deemed to impose any duty or obligation on the Guarantee Trustee
     to perform any act or acts or exercise any right, power, duty or
     obligation conferred or imposed on it in any jurisdiction in which
     it shall be illegal, or in which the Guarantee Trustee shall be
     unqualified or incompetent in accordance with applicable law, to
     perform any such act or acts or to exercise any such right, power,
     duty or obligation. No permissive power or authority available to
     the Guarantee Trustee shall be construed to be a duty to act in
     accordance with such power and authority.

     Section 3.3.    Indemnity.

     The Guarantor agrees to indemnify the Guarantee Trustee (which for
purposes of this Section 3.3 shall include its directors, officers,
employees and agents) for, and to hold the Guarantee Trustee harmless
against, any loss, liability or expense incurred without negligence,
willful misconduct or bad faith on the part of the Guarantee Trustee,
arising out of or in connection with the acceptance or administration of
this Guarantee Agreement, including the reasonable costs and expenses of
defending

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<PAGE>

against any claim or liability in connection with the exercise or
performance of any of its powers or duties hereunder.  The Guarantee
Trustee will not claim or exact any lien or charge on any Guarantee
Payments as a result of any amount due to it under this Guarantee
Agreement.

     Section 3.4.    Expenses.

     The Guarantor shall from time to time reimburse the Guarantee
Trustee for its reasonable expenses and costs (including reasonable
attorneys' or agents' fees) incurred in connection with the performance
of its duties hereunder.

                             ARTICLE IV

                         GUARANTEE TRUSTEE

     Section 4.1.    Guarantee Trustee; Eligibility.

          (a)   There shall at all times be a Guarantee Trustee
     which shall:

                (i) not be an Affiliate of the Guarantor; and

                (ii) be a Person that is eligible pursuant to the
          Trust Indenture Act to act as such and has a combined
          capital and surplus of at least $50,000,000, and shall be a
          corporation meeting the requirements of Section 310(a) of
          the Trust Indenture Act. If such corporation publishes
          reports of condition at least annually, pursuant to law or
          to the requirements of the supervising or examining
          authority, then, for the purposes of this Section and to the
          extent permitted by the Trust Indenture Act, the combined
          capital and surplus of such corporation shall be deemed to
          be its combined capital and surplus as set forth in its most
          recent report of condition so published.

          (b)   If at any time the Guarantee Trustee shall cease to
     be eligible to so act under Section 4.1(a), the Guarantee Trustee
     shall immediately resign in the manner and with the effect set out
     in Section 4.2(b).

          (c)   If the Guarantee Trustee has or shall acquire any
     "conflicting interest" within the meaning of Section 310(b) of the
     Trust Indenture Act, the Guarantee Trustee and Guarantor shall in
     all respects comply with the provisions of Section 310(b) of the
     Trust Indenture Act.

     Section 4.2.    Appointment, Removal and Resignation of the
                     Guarantee Trustee.

          (a)   No resignation or removal of the Guarantee Trustee
     and no appointment of a Successor Guarantee Trustee pursuant to
     this Article IV shall become effective until the acceptance of
     appointment by the Successor Guarantee Trustee by written
     instrument executed by the Successor Guarantee Trustee and
     delivered to the Holders and the Guarantee Trustee.

          (b)   Subject to Section 4.2(a), a Guarantee Trustee may
     resign at any time by giving written notice thereof to the
     Holders. The Guarantee Trustee shall appoint a successor by
     requesting from at least three Persons meeting the eligibility
     requirements such Person's expenses and charges to serve as the
     Guarantee Trustee, and selecting the Person who agrees to the
     lowest expenses and charges.  If the instrument of acceptance by
     the Successor Guarantee

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     Trustee shall not have been delivered to the Guarantee Trustee
     within 60 days after the giving of such notice of resignation,
     the Guarantee Trustee may petition, at the expense of the Guarantor,
     any court of competent jurisdiction for the appointment of a
     Successor Guarantee Trustee.

          (c)   The Guarantee Trustee may be removed for Cause at
     any time by Act of the Holders of at least a Majority in
     Liquidation Amount of the Preferred Securities, delivered to the
     Guarantee Trustee.

          (d)   If a resigning Guarantee Trustee shall fail to
     appoint a successor, or if a Guarantee Trustee shall be removed
     or become incapable of acting as Guarantee Trustee, or if any
     vacancy shall occur in the office of any Guarantee Trustee for any
     cause, the Holders of the Preferred Securities, by Act of the
     Holders of record of not less than 25% in aggregate Liquidation
     Amount of the Preferred Securities then outstanding delivered to
     such Guarantee Trustee, shall promptly appoint a successor
     Guarantee Trustee.  If no Successor Guarantee Trustee shall have
     been so appointed by the Holders of the Preferred Securities and
     such appointment accepted by the Successor Guarantee Trustee, any
     Holder, on behalf of himself and all others similarly situated,
     may petition any court of competent jurisdiction for the
     appointment of a Successor Guarantee Trustee.

                             ARTICLE V

                             GUARANTEE

     Section 5.1.    Guarantee.

     The Guarantor irrevocably and unconditionally agrees to pay in
full on a subordinated basis as set forth in Section 6.1 hereof to the
Holders the Guarantee Payments (without duplication of amounts
theretofore paid by or on behalf of the Issuer Trust), as and when due,
regardless of any defense, right of set-off or counterclaim which the
Issuer Trust may have or assert, except the defense of payment. The
Guarantor's obligation to make a Guarantee Payment may be satisfied by
direct payment of the required amounts by the Guarantor to the Holders
or by causing the Issuer Trust to pay such amounts to the Holders. The
Guarantor shall give prompt written notice to the Guarantee Trustee in
the event it makes any direct payment hereunder.

     Section 5.2.    Waiver of Notice and Demand.

     The Guarantor hereby waives notice of acceptance of the Guarantee
Agreement and of any liability to which it applies or may apply,
presentment, demand for payment, any right to require a proceeding first
against the Guarantee Trustee, the Issuer Trust or any other Person
before proceeding against the Guarantor, protest, notice of nonpayment,
notice of dishonor, notice of redemption and all other notices and
demands.

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     Section 5.3.    Obligations Not Affected.

     The obligations, covenants, agreements and duties of the Guarantor
under this Guarantee Agreement shall in no way be affected or impaired
by reason of the happening from time to time of any of the following:

          (a)   the release or waiver, by operation of law or
     otherwise, of the performance or observance by the Issuer Trust of
     any express or implied agreement, covenant, term or condition
     relating to the Preferred Securities to be performed or observed
     by the Issuer Trust;

          (b)   the extension of time for the payment by the Issuer
     Trust of all or any portion of the Distributions (other than an
     extension of time for payment of Distributions that results from
     the extension of any interest payment period on the Junior
     Subordinated Debentures as so provided in the Indenture),
     Redemption Price, Liquidation Distribution or any other sums
     payable under the terms of the Preferred Securities or the
     extension of time for the performance of any other obligation
     under, arising out of, or in connection with, the Preferred
     Securities;

          (c)   any failure, omission, delay or lack of diligence on
     the part of the Holders to enforce, assert or exercise any right,
     privilege, power or remedy conferred on the Holders pursuant to
     the terms of the Preferred Securities, or any action on the part
     of the Issuer Trust granting indulgence or extension of any kind;

          (d)   the voluntary or involuntary liquidation,
     dissolution, sale of any collateral, receivership, insolvency,
     bankruptcy, assignment for the benefit of creditors,
     reorganization, arrangement, composition or readjustment of debt
     of, or other similar proceedings affecting, the Issuer Trust or
     any of the assets of the Issuer Trust;

          (e)   any invalidity of, or defect or deficiency in, the
     Preferred Securities;

          (f)   any failure or omission to receive any regulatory
     approval or consent required in connection with the Preferred
     Securities (or the Common Securities issued by the Issuer
     Trust), including, without limitation, the failure to receive any
     approval of the Board of Governors of the Federal Reserve System
     required for the redemption of the Preferred Securities;

          (g)   the settlement or compromise of any obligation
     guaranteed hereby or hereby incurred; or

          (h)   any other circumstance whatsoever that might
     otherwise constitute a legal or equitable discharge or defense of
     a guarantor (other than payment of the underlying obligation), it
     being the intent of this Section 5.3 that the obligations of the
     Guarantor hereunder shall be absolute and unconditional under any
     and all circumstances.

     There shall be no obligation of the Holders to give notice to, or
obtain the consent of, the Guarantor with respect to the happening of
any of the foregoing.

     Section 5.4.    Rights of Holders.

     The Guarantor expressly acknowledges that:

          (a)   this Guarantee Agreement will be deposited with the
     Guarantee Trustee to be held for the benefit of the Holders;

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          (b)   the Guarantee Trustee has the right to enforce this
     Guarantee Agreement on behalf of the Holders;

          (c)   the Holders of a Majority in Liquidation Amount of
     the Preferred Securities have the right to direct the time,
     method and place of conducting any proceeding for any remedy
     available to the Guarantee Trustee in respect of this Guarantee
     Agreement or exercising any trust or power conferred upon the
     Guarantee Trustee under this Guarantee Agreement; and

          (d)   any Holder may institute a legal proceeding directly
     against the Guarantor to enforce its rights under this Guarantee
     Agreement, without first instituting a legal proceeding against
     the Guarantee Trustee, the Issuer Trust or any other Person.

     Section 5.5.    Guarantee of Payment.

     This Guarantee Agreement creates a guarantee of payment and not of
collection. This Guarantee Agreement will not be discharged except by
payment of the Guarantee Payments in full (without duplication of
amounts theretofore paid by the Issuer Trust) or upon the distribution
of Junior Subordinated Debentures to Holders as provided in the Trust
Agreement.

     Section 5.6.    Subrogation.

     The Guarantor shall be subrogated to all rights (if any) of the
Holders against the Issuer Trust in respect of any amounts paid to the
Holders by the Guarantor under this Guarantee Agreement; provided,
however, that the Guarantor shall not (except to the extent required by
mandatory provisions of law) be entitled to enforce or exercise any
rights which it may acquire by way of subrogation or any indemnity,
reimbursement or other agreement, in all cases as a result of payment
under this Guarantee Agreement, if at the time of any such payment, any
amounts are due and unpaid under this Guarantee Agreement. If any amount
shall be paid to the Guarantor in violation of the preceding sentence,
the Guarantor agrees to hold such amount in trust for the Holders and to
pay over such amount to the Holders.

     Section 5.7.    Independent Obligations.

     The Guarantor acknowledges that its obligations hereunder are
independent of the obligations of the Issuer Trust with respect to the
Preferred Securities and that the Guarantor shall be liable as principal
and as debtor hereunder to make Guarantee Payments pursuant to the terms
of this Guarantee Agreement notwithstanding the occurrence of any event
referred to in subsections (a) through (h), inclusive, of Section 5.3
hereof.

                             ARTICLE VI

                    COVENANTS AND SUBORDINATION

     Section 6.1.    Subordination.

     This Guarantee Agreement will constitute an unsecured obligation
of the Guarantor and will rank subordinate and junior in right of
payment to all Senior Indebtedness of the Guarantor to the extent and in
the manner set forth in the Indenture with respect to the Junior
Subordinated Debentures, and the provisions of Article XIII of the
Indenture will apply, mutatis mutandis, to the obligations of the
Guarantor hereunder. The obligations of the Guarantor hereunder do not
constitute Senior Indebtedness of the Guarantor.

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     Section 6.2.    Pari Passu Guarantees.

     The obligations of the Guarantor under this Guarantee Agreement
shall rank pari passu with any similar guarantee agreements issued by
the Guarantor on behalf of the holders of preferred or capital
securities issued by the Issuer Trust and with any other security,
guarantee or other obligation that is expressly stated to rank pari
passu with the obligations of the Guarantor under this Guarantee
Agreement.

                            ARTICLE VII

                            TERMINATION

     Section 7.1.    Termination.

     This Guarantee Agreement shall terminate and be of no further
force and effect upon (a) full payment of the Redemption Price of all
Preferred Securities, (b) the distribution of Junior Subordinated
Debentures to the Holders in exchange for all of the Preferred
Securities or (c) full payment of the amounts payable in accordance with
Article IX of the Trust Agreement upon liquidation of the Issuer Trust.
Notwithstanding the foregoing, this Guarantee Agreement will continue to
be effective or will be reinstated, as the case may be, if at any time
any Holder is required to repay any sums paid with respect to the
Preferred Securities or this Guarantee Agreement.

                            ARTICLE VIII

                           MISCELLANEOUS

     Section 8.1.    Successors and Assigns.

     All guarantees and agreements contained in this Guarantee
Agreement shall bind the successors, assigns, receivers, trustees and
representatives of the Guarantor and shall inure to the benefit of the
Holders of the Preferred Securities then outstanding. Except in
connection with a consolidation, merger or sale involving the Guarantor
that is permitted under Article VIII of the Indenture and pursuant to
which the assignee agrees in writing to perform the Guarantor's
obligations hereunder, the Guarantor shall not assign its obligations
hereunder, and any purported assignment that is not in accordance with
these provisions shall be void.

     Section 8.2.    Amendments.

     Except with respect to any changes that do not materially
adversely affect the rights of the Holders (in which case no consent of
the Holders will be required), this Guarantee Agreement may only be
amended with the prior approval of the Holders of not less than a
Majority in Liquidation Amount of the Preferred Securities. The
provisions of Article VI of the Trust Agreement concerning meetings of
the Holders shall apply to the giving of such approval.

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     Section 8.3.    Notices.

     Any notice, request or other communication required or permitted
to be given hereunder shall be in writing, duly signed by the party
giving such notice, and delivered, telecopied (confirmed by delivery of
the original) or mailed by first class mail as follows:

          (a)   if given to the Guarantor, to the address or
     telecopy number set forth below or such other address or telecopy
     number or to the attention of such other Person as the Guarantor
     may give notice to the Holders:

                     Allegiant Bancorp, Inc.
                     2122 Kratky Road
                     St. Louis, Missouri 63114
                     Facsimile No.: (314) 692-8500
                     Attention:  President

          (b)   if given to the Issuer Trust, in care of the
     Guarantee Trustee, at the Issuer Trust's (and the Guarantee
     Trustee's) address set forth below or such other address or
     telecopy number or to the attention of such other Person as the
     Guarantee Trustee on behalf of the Issuer Trust may give notice to
     the Holders:

                     Allegiant Capital Trust I
                     c/o Allegiant Bancorp, Inc.
                     2122 Kratky Road
                     St. Louis, Missouri 63114
                     Facsimile No.: (314) 692-8500
                     Attention:  President

          with a copy to:

                     Bankers Trust Company
                     Four Albany Street - 4th Floor
                     New York, New York 10006
                     Facsimile No.: (212) 250-6961
                     Attention: Corporate Trust and Agency Group;
                                  Corporate Market Services

          (c)   if given to the Guarantee Trustee:

                     Bankers Trust Company
                     Four Albany Street - 4th Floor
                     New York, New York 10006
                     Facsimile No.: (212) 250-6961
                     Attention: Corporate Trust and Agency Group
                                  Corporate Market Services

          (d)   if given to any Holder, at the address set forth on
     the books and records of the Issuer Trust.

     All notices hereunder shall be deemed to have been given when
received in person, telecopied with receipt confirmed, or mailed by
first class mail, postage prepaid, except that if a notice or other
document is refused delivery or cannot be delivered because of a changed
address of which no notice was given, such notice or other document
shall be deemed to have been delivered on the date of such refusal or
inability to deliver.

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     Section 8.4.    Benefit.

     This Guarantee Agreement is solely for the benefit of the Holders
and is not separately transferable from the Preferred Securities.

     Section 8.5.    Interpretation.

     In this Guarantee Agreement, unless the context otherwise
requires:

          (a)   capitalized terms used in this Guarantee Agreement
     but not defined in the preamble hereto have the respective
     meanings assigned to them in Section 1.1;

          (b)   a term defined anywhere in this Guarantee Agreement
     has the same meaning throughout;

          (c)   all references to "the Guarantee Agreement" or "this
     Guarantee Agreement" are to this Guarantee Agreement as modified,
     supplemented or amended from time to time;

          (d)   all references in this Guarantee Agreement to
     Articles and Sections are to Articles and Sections of this
     Guarantee Agreement unless otherwise specified;

          (e)   a term defined in the Trust Indenture Act has the
     same meaning when used in this Guarantee Agreement unless
     otherwise defined in this Guarantee Agreement or unless the
     context otherwise requires;

          (f)   a reference to the singular includes the plural and
     vice versa;

          (g)   the masculine, feminine or neuter genders used
     herein shall include the masculine, feminine and neuter genders;

          (h)   the words "include," "includes" and "including"
     shall be deemed to be followed by the phrase "without limitation";
     and

          (i)   the words "herein," "hereof" and "hereunder" and
     other words of similar import refer to this Guarantee Agreement as
     a whole and not to any particular Article, Section or other
     subdivision.

     Section 8.6.    Governing Law.

     This Guarantee Agreement shall be governed by and construed and
interpreted in accordance with the laws of the State of New York without
regard to the conflict of law principles thereof.

     Section 8.7.    Counterparts.

     This instrument may be executed in any number of counterparts,
each of which so executed shall be deemed to be an original, but all
such counterparts shall together constitute but one and the same
instrument.

         [SIGNATURES APPEAR ON THE FOLLOWING PAGE]

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     THIS GUARANTEE AGREEMENT is executed as of the day and year first
above written.

                          ALLEGIANT BANCORP, INC., as Guarantor


                          By: /s/ Shaun R. Hayes
                             -----------------------------------------
                          Shaun R. Hayes
                          President and Chief Executive Officer


                          BANKERS TRUST COMPANY, as Guarantee Trustee
                          and in its individual capacity


                          By: /s/ Ednora G. Linares
                             -----------------------------------------
                          Name: Ednora G. Linares
                          Title: Assistant Vice President


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