<PAGE>

                                                                   EXHIBIT 4.10
                                                                   ------------

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















                              GUARANTEE AGREEMENT


                                    Between


                            ALLEGIANT BANCORP, INC.
                                 (as Guarantor)


                                      and


                             BANKERS TRUST COMPANY
                             (as Guarantee Trustee)


                                  dated as of


                                 _____ __, 2001











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




<PAGE>
<PAGE>

                         ALLEGIANT CAPITAL TRUST II

          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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<PAGE>

<TABLE>
                                  TABLE OF CONTENTS

<S>                                                                                <C>
ARTICLE I  DEFINITIONS ...........................................................  2
  Section 1.1.  Definitions ......................................................  2

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 ........................  6
  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 ..................  7
  Section 3.1.  Powers and Duties of the Guarantee Trustee .......................  7
  Section 3.2.  Certain Rights of Guarantee Trustee ..............................  8
  Section 3.3.  Indemnity ........................................................ 10
  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 .... 11

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

ARTICLE VI  COVENANTS AND SUBORDINATION .......................................... 14
  Section 6.1.  Subordination .................................................... 14
  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 ....................................................... 15
  Section 8.3.  Notices .......................................................... 15
  Section 8.4.  Benefit .......................................................... 16
  Section 8.5.  Interpretation ................................................... 16
  Section 8.6.  Governing Law .................................................... 17
  Section 8.7.  Counterparts ..................................................... 17
</TABLE>




                                    - i -

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<PAGE>

                             GUARANTEE AGREEMENT


         THIS GUARANTEE AGREEMENT, dated as of ________, 2001 (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 _______, 2001, 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 $34,500,000 aggregate Liquidation Amount
(as defined herein) of its _____% Preferred Securities, Liquidation Amount
$25 per preferred security (the "Preferred Securities") (including up to
180,000 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 _____ __, 2031 (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.



<PAGE>
<PAGE>

                                  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.

     "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 _____ __, 2001 and payable quarterly in arrears on March
31, June 30, September 30 and December 31 of each year, commencing December
31, 2001 at the annual rate of _____% 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.

                                   - 2 -

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<PAGE>

     "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 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 _____
__, 2001, 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.

                                   - 3 -

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<PAGE>

     "Liquidation Amount" means the stated amount of $25 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.

     "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


                                   - 4 -

<PAGE>
<PAGE>

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.

     "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,
2002, 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.

                                   - 5 -

<PAGE>
<PAGE>

     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.

     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.

                                   - 6 -

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<PAGE>

                                 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 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

                                   - 7 -

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<PAGE>

              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 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

                                   - 8 -

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<PAGE>

         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
         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

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     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 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.

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     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 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.

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<PAGE>

     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.

     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.

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     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;

         (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.

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                                 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.

     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, including, without
limitation, the guarantees and obligations of Guarantor pursuant to the
Guarantee Agreement, dated August 2, 1999, between Guarantor and Guarantee
Trustee, issued in connection with the issuance of the 9.875% cumulative
trust preferred securities and trust common securities.

                                 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

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<PAGE>

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.

     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 II
                  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) 250-2500
                  Attention: Corporate Trust and Agency Group;
                               Corporate Market Services

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         (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.

     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;

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         (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]


                                   - 17 -

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

                              ALLEGIANT BANCORP, INC., as Guarantor


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


                              BANKERS TRUST COMPANY, as Guarantee and
                              in its individual capacity


                              By:
                                 -------------------------------------
                              Name:
                              Title:


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