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                                                                    EXHIBIT 10.1

                             STOCK OPTION AGREEMENT

         This STOCK OPTION AGREEMENT (this "Agreement") is entered into this
13th day of June, 2001, between F.N.B. CORPORATION, a Florida corporation
having its principal office located in Naples, Florida ("FNB"), and PROMISTAR
FINANCIAL CORPORATION, a Pennsylvania corporation having its principal office
located in Johnstown, Pennsylvania ("Promistar").

                                   WITNESSETH:

         WHEREAS, FNB and Promistar have entered into an Agreement and Plan of
Merger of even date herewith (the "Merger Agreement"), which agreement has been
executed by the parties hereto prior to the execution of this Agreement; and

         WHEREAS, as a condition and inducement to FNB's pursuit of the
transactions contemplated by the Merger Agreement and in consideration therefor,
Promistar has agreed to grant FNB the Option (as hereinafter defined).

         NOW, THEREFORE, in consideration of the foregoing and the mutual
covenants and agreements set forth herein and in the Merger Agreement, the
parties hereto agree as follows:

         1.  (a) Promistar hereby grants to FNB an irrevocable option (the
"Option") to purchase, subject to the terms hereof, up to 2,975,830 authorized
but unissued fully paid and nonassessable Common Shares, $5.00 par value, of
Promistar ("Common Shares"), at a price per Common Share equal to $17.306 (as
adjusted as set forth herein, the "Option Price"); provided, that in no event
shall the number of Common Shares for which this Option is exercisable, when
combined with the Promistar Common Shares beneficially owned at such time by
FNB, exceed 19.9% of the issued and outstanding Common Shares. The number of
Common Shares that may be received upon the exercise of the Option and the
Option Price are subject to adjustment as herein set forth.

              (b) In the event that any additional Common Shares are issued or
otherwise become outstanding after the date of this Agreement (other than
pursuant to this Agreement), the number of Common Shares subject to the Option
shall be increased so that, after such issuance, it equals 19.9% of the number
of Common Shares then issued and outstanding including Common Shares
beneficially owned by FNB, but without giving effect to any Shares subject or
issued pursuant to the Option. Nothing contained in this Section 1(b) or
elsewhere in this Agreement shall be deemed to authorize Promistar or FNB to
breach any provision of the Merger Agreement.

         2.   (a) Subject to compliance with applicable laws and regulations,
the Holder (as hereinafter defined) may exercise the Option, notwithstanding the
provisions of the Confidentiality Agreements (as defined in the Merger
Agreement) in whole or part, if, but only if, both an Initial Triggering Event
(as hereinafter defined) and a Subsequent Triggering Event (as hereinafter
defined) shall have occurred prior to the occurrence of an Exercise Termination
Event (as hereinafter defined). Each of the following shall be an Exercise
Termination Event: (i) the Effective Time (as defined in the Merger Agreement)
of the Merger; (ii) termination of the Merger Agreement in accordance with the
provisions thereof if such termination occurs prior to the occurrence of an
Initial Triggering Event (other than termination due to


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the failure of FNB to satisfy a condition to closing; (iii) the passage of 12
months (or such longer period as provided in Section 9) after termination of the
Merger Agreement if such termination follows the occurrence of an Initial
Triggering Event; or (iv) such other date as to which the Holder and Promistar
agree. The term "Holder" shall mean the holder or holders of the Option. The
rights set forth in Section 7 shall terminate when the right to exercise the
Option terminates (other than as a result of a complete exercise of the Option)
as set forth herein.

              (b) The term "Initial Triggering Event" shall mean any of the
following events or transactions occurring after the date hereof:

                  (i)     Promistar or any of its Subsidiaries (as hereinafter
defined) (each a "Promistar Subsidiary"), without having received FNB's prior
written consent, shall have entered into an agreement to engage in an
Acquisition Transaction (as hereinafter defined) with any person (the term
"person" for purposes of this Agreement having the meaning assigned thereto in
Sections 3(a)(9) and 13(d)(3) of the Securities Exchange Act of 1934 (the "1934
Act"), and the rules and regulations thereunder) other than FNB or any of its
Subsidiaries (each a "FNB Subsidiary") or the Board of Directors of Promistar
shall have recommended that the shareholders of Promistar approve or accept any
Acquisition Transaction other than as contemplated by the Merger Agreement or
this Agreement. For purposes of this Agreement, (a) "Acquisition Transaction"
shall mean (x) a merger or consolidation, or any similar transaction, involving
Promistar or any Significant Subsidiary (as defined in Rule 1-02 of Regulation
S-X promulgated by the Securities and Exchange Commission (the "SEC")) of
Promistar, (y) a purchase, lease or other acquisition of all or substantially
all of the assets or deposits of Promistar or any Significant Subsidiary of
Promistar, or (z) a purchase or other acquisition (including by way of merger,
consolidation, share exchange or otherwise) of securities representing 15% or
more of the voting power of Promistar or any Significant Subsidiary of
Promistar, and (b) "Subsidiary" shall have the meaning set forth in Rule 12b-2
under the 1934 Act;

                  (ii)    Any person (excluding the officers and directors of
Promistar) other than FNB, any FNB Subsidiary or any Promistar Subsidiary acting
in a fiduciary capacity shall have acquired beneficial ownership or the right to
acquire beneficial ownership of 15% or more of the outstanding Common Shares
(the term "beneficial ownership" for purposes of this Agreement having the
meaning assigned thereto in Section 13(d) of the 1934 Act, and the rules and
regulations thereunder);

                  (iii)   The shareholders of the Promistar shall not have
approved the transactions contemplated by the Merger Agreement at the meeting
held for that purpose or any adjustment thereof, or such meeting shall not have
been held or shall have been canceled prior to termination of the Merger
Agreement, in either case, after Promistar's Board of Directors shall have
withdrawn or modified (or publicly announced its intention to withdraw or modify
or interest in withdrawing or modifying) its recommendation that the
shareholders of Promistar approve the transactions contemplated by the Merger
Agreement, or Promistar or any Promistar Subsidiary, without having received
FNB's prior written consent, shall have authorized, recommended, proposed (or
publicly announced its intention to authorize, recommend or propose or interest
in authorizing, recommending or proposing) an agreement to engage in an
Acquisition Transaction, with any person other than FNB or a FNB Subsidiary;

                  (iv)    Any person other than FNB or any FNB Subsidiary shall
have made a bona fide proposal to Promistar or its shareholders to engage in an
Acquisition Transaction, which proposal has an economic value equivalent to or
in excess of that of FNB.


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                  (v)   Promistar shall have willfully and materially breached
any material covenant or obligation contained in the Merger Agreement in
anticipation of engaging in an Acquisition Transaction, and such breach would
entitle FNB to terminate the Merger Agreement; or

                  (vi)  Any person other than FNB or any FNB Subsidiary, other
than in connection with a transaction to which FNB has given its prior written
consent, shall have filed an application or notice with the Federal Reserve
Board or other federal or state bank regulatory authority, which application or
notice has been accepted for processing, for approval to engage in an
Acquisition Transaction.

              (c) The term "Subsequent Triggering Event" shall mean any of the
following events or transactions occurring after the date hereof:

                  (i)  The acquisition by any person of beneficial ownership of
25% or more of the then outstanding Common Shares; or

                  (ii) The occurrence of the Initial Triggering Event described
in clause (i) of subsection (b) of this Section 2, except that the percentage
referred to in clause (z) shall be 25%.

              (d) Promistar shall notify FNB promptly in writing of the
occurrence of any Initial Triggering Event or Subsequent Triggering Event
(together, a "Triggering Event"), it being understood that the giving of such
notice by Promistar shall not be a condition to the right of the Holder to
exercise the Option.

              (e) No shares shall be issued pursuant to the exercise of this
Option if (i) at the time of the Initial Triggering Event and at the time of
exercise, FNB is in material breach under the Merger Agreement, or (ii) a
preliminary or permanent injunction has been issued by a court of proper
jurisdiction with respect to this Option or the Merger Agreement or the
transactions contemplated hereby or thereby.

              (f) In the event the Holder is entitled to and wishes to exercise
the Option, it shall send to Promistar a written notice prior to an Exercise
Termination Event (the date of which being herein referred to as the "Notice
Date") specifying (i) the total number of shares it will purchase pursuant to
such exercise and (ii) a place and date not earlier than three business days nor
later than 10 business days from the Notice Date for the closing of such
purchase (the "Closing Date"); provided that if prior notification to or
approval of the Federal Reserve Board or any other regulatory agency is required
in connection with such purchase, the Holder shall promptly file the required
notice or application for approval, shall promptly notify the Promistar of such
filing, and shall expeditiously process the same and the period of time that
otherwise would run pursuant to this sentence shall run instead from the date on
which any required notification periods have expired or been terminated or such
approvals have been obtained and any requisite waiting period or periods shall
have passed. Any exercise of the Option shall be deemed to occur on the Notice
Date relating thereto.

              (g) At the closing referred to in subsection (e) of this Section
2, the Holder shall pay to Promistar the aggregate purchase price for the Common
Shares purchased pursuant to the exercise of the Option in immediately available
funds by wire transfer to a bank account designated by Promistar, provided that
failure or refusal of Promistar to designate such a bank account shall not
preclude the Holder from exercising the Option.


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              (h) At such closing, simultaneously with the delivery of
immediately available funds as provided in subsection (f) of this Section 2,
Promistar shall deliver to the Holder a certificate or certificates representing
the number of Common Shares purchased by the Holder and, if the Option should be
exercised in part only, a new Option evidencing the rights of the Holder thereof
to purchase the balance of the shares purchasable thereunder. In addition, the
Holder shall provide to Promistar a letter agreeing that Holder will not offer
to sell or dispose of such shares in violation of applicable law or this
Agreement

              (i) Certificates for Common Shares delivered at a closing
hereunder may be endorsed with a restrictive legend that shall read
substantially as follows:

                  "The transfer of the shares represented by this certificate is
                  subject to certain provisions of an agreement between the
                  registered holder hereof and Promistar and to resale
                  restrictions arising under the Securities Act of 1933, as
                  amended. A copy of such agreement is on file at the principal
                  office of Promistar and will be provided to the holder hereof
                  without charge upon receipt by Promistar of a written request
                  therefor."

                  It is understood and agreed that: (1) the reference to the
                  resale restrictions of the Securities Act of 1933 (the "1933
                  Act") in the above legend shall be removed by delivery of
                  substitute certificate(s) without such reference if the Holder
                  shall have delivered to Promistar a copy of a letter from the
                  staff of the SEC, or an opinion of counsel, in form and
                  substance satisfactory to Promistar, to the effect that such
                  legend is not required for purposes of the 1933 Act; (ii) the
                  reference to the provisions of this Agreement in the above
                  legend shall be removed by delivery of substitute
                  certificate(s) without such reference if the shares have been
                  sold or transferred in compliance with the provisions of this
                  Agreement and under circumstances that do not require the
                  retention of such reference; and (iii) the legend shall be
                  removed in its entirety if the conditions in the proceeding
                  clauses (i) and (ii) are both satisfied. In addition, such
                  certificates shall bear any other legend as may be required by
                  law.

              (j) Upon the giving by the Holder to Promistar of the written
notice of exercise of the Option provided for under subsection (e) of this
Section 2 and the tender of the applicable purchase price in immediately
available funds the Holder shall be deemed to be the holder of record of the
Common Shares issuable upon such exercise, notwithstanding that the stock
transfer books of Promistar shall then be closed or that certificates
representing such Common Shares shall not then be actually delivered to the
Holder. Promistar shall pay all expenses, and any and all United States federal,
state and local taxes and other charges that may be payable in connection with
the preparation, issue and delivery of stock certificates under this Section 2
in the name of the Holder or its assignee, transferee or designee.

         3. Promistar agrees: (a) that it shall at all times maintain, free from
preemptive rights, sufficient authorized but unissued or treasury shares of
Common Shares so that the Option may be exercised without additional
authorization of Common Shares after giving effect to all other options,
warrants, convertible securities and other rights to purchase Common Shares; (b)
that it will not, by charter amendment or through reorganization, consolidation,
merger, dissolution or sale of assets, or by any other voluntary act, avoid or
seek to avoid the observance or performance of any of the covenants,
stipulations


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or conditions to be observed or performed hereunder by Promistar; (c) promptly
to take all action as may from time to time be required (including (i) complying
with all premerger notification, reporting and waiting period requirements
specified in 15 U.S.C. Section 18a and regulations promulgated thereunder and
(ii) in the event, under the Bank Holding Company Act of 1956, as amended, or
any state or other federal banking law, prior approval of or notice to the
Federal Reserve Board or to any state or other federal regulatory authority is
necessary before the Option may be exercised, cooperating fully with the Holder
in preparing such applications or notices and providing such information to the
Federal Reserve Board or such state or other federal regulatory authority as
they may require) in order to permit the Holder to exercise the Option and
Promistar duly and effectively to issue Common Shares pursuant hereto; and (d)
promptly to take all action provided herein to protect the rights of the Holder
against dilution as set forth in Section 5 hereof.

         4. This Agreement (and the Option granted hereby) are exchangeable,
without expense, at the option of the Holder, upon presentation and surrender of
this Agreement at the principal office of Promistar, for other Agreements
providing for Options of different denominations entitling the holder thereof to
purchase, on the same terms and subject to the same conditions as are set forth
herein, in the aggregate the same number of Common Shares purchasable hereunder.
The terms "Agreement" and "Option" as used herein include any Agreements and
related Options for which this Agreement (and the Option granted hereby) may be
exchanged. Upon receipt by Promistar of evidence reasonably satisfactory to it
of the loss, theft, destruction or mutilation of this Agreement, and (in the
case of loss, theft or destruction) of reasonably satisfactory indemnification,
and upon surrender and cancellation of this Agreement, if mutilated, Promistar
will execute and deliver a new Agreement of like tenor and date.

         5. The number of Common Shares purchasable upon the exercise of the
Option shall be subject to adjustment from time to time as provided in this
Section 5.

            (a) In the event of any change in Common Shares by reason of stock
dividends, splitups, mergers, recapitalizations, combinations, subdivisions,
conversions, exchanges of shares or the like, the type and number of Common
Shares purchasable upon exercise hereof shall be appropriately adjusted and
proper provision shall be made so that, in the event that any additional Common
Shares are to be issued or otherwise become outstanding as a result of any such
change (other than pursuant to an exercise of the Option), the number of Common
Shares that remain subject to the Option shall be increased so that, after such
issuance and together with Common Shares previously issued pursuant to the
exercise of the Option (together with the number of Shares previously issued
under this Option and the number of Shares otherwise beneficially owned by FNB)
(as adjusted on account of any of the foregoing changes in the Common Shares),
it equals 19.9% of the number of Common Shares then issued and outstanding.

            (b) Whenever the number of Common Shares purchasable upon exercise
hereof is adjusted as provided in this Section 5, the Option Price shall be
adjusted by multiplying the Option Price by a fraction, the numerator of which
shall be equal to the number of Common Shares purchasable prior to the
adjustment and the denominator of which shall be equal to the number of Common
Shares purchasable after the adjustment.

         6. Upon the occurrence of a Subsequent Triggering Event that occurs
prior to an Exercise Termination Event, Promistar shall, at the request of FNB
delivered prior to an Exercise Termination Event (or such later period as
provided in Section 9) (whether on its own behalf or on behalf of any subsequent
holder of this Option (or part thereof) or any of the Common Shares issued
pursuant hereto), promptly prepare, file and keep current a registration
statement under the 1933 Act covering any shares


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issued and issuable pursuant to this Option and shall use its best efforts to
cause such registration statement to become effective and remain current in
order to permit the sale or other disposition of any Common Shares issued upon
total or partial exercise of this Option ("Option Shares") in accordance with
any plan of disposition requested by FNB. Promistar will use its best efforts to
cause such registration statement first to become effective and then to remain
effective for such period not in excess of 120 days from the day such
registration statement first becomes effective or such shorter time as may be
reasonably necessary to effect such sales or other dispositions. FNB shall have
the right to demand two such registrations. The first demand registration
effected under this Section 6 shall be at Promistar's expense except for
underwriting commissions and the fees and expenses of FNB's counsel attributable
to the registration of the Common Shares. The second demand registration shall
be at FNB's expense. In addition, if at any time after the occurrence of a
Subsequent Triggering Event that occurs prior to an Exercise Termination Event,
Promistar proposes to register any of its equity securities under the 1933 Act,
whether for sale for its own account or for the account of any other person, on
a form and in a manner which would permit registration of the Common Shares
issued pursuant hereto for sale to the public under the 1933 Act, it will each
such time give prompt written notice to FNB of its intention to do so,
describing such securities and specifying the form and manner and the other
relevant facts involved in such proposed registration, and upon the written
request of FNB delivered to the Company within 10 business days after the giving
of any such notice (which request shall specify the Common Shares intended to be
disposed of and the intended method or methods of disposition thereof),
Promistar will use its best efforts to effect the registration under the 1933
Act of all Common Shares which Promistar has been so requested to register by
FNB, to the extent requisite to permit the disposition of the Common Shares in
accordance with the intended methods thereof as specified by FNB. Promistar
shall be obligated to effect only one such piggy-back registration pursuant to
this Section 6. FNB shall pay such incremental expenses incurred by Promistar in
connection with registering the Common Shares requested to be registered by FNB
pursuant to its piggy-back registration rights under this Section 6, which
expenses are in addition to the expenses that Promistar would have otherwise
incurred in registering equity securities under the 1933 Act. The foregoing
notwithstanding, if, at the time of any request by FNB for registration of
Option Shares as provided above, Promistar has initiated discussions with
investment bankers concerning, or is in registration with respect to an
underwritten public offering of Common Shares, and if in the good faith judgment
of the managing underwriter or managing underwriters, or, if none, the sole
underwriter or underwriters, of such offering the inclusion of the Option Shares
would interfere with the successful marketing of the Common Shares offered by
Promistar, the number of Option Shares otherwise to be covered in the
registration statement contemplated hereby may be reduced; provided, however,
that after any such required reduction the number of Option Shares to be
included in such offering for the account of the Holder shall constitute at
least 25% of the total number of shares to be sold by the Holder and Promistar
in the aggregate; and provided further, however, that if such reduction occurs,
then the Promistar shall file a registration statement for the balance as
promptly as practical thereafter as to which no reduction pursuant to this
Section 6 shall be permitted or occur and the Holder shall thereafter be
entitled to one additional registration at Holder's expense. Each such Holder
shall provide all information reasonably requested by Promistar for inclusion in
any registration statement to be filed hereunder. If requested by any such
Holder in connection with such registration, Promistar shall become a party to
any underwriting agreement relating to the sale of such shares, but only to the
extent of obligating itself in respect of representations, warranties,
indemnities and other agreements customarily included in such underwriting
agreements for Promistar. In any such registration, Promistar and FNB shall
agree to indemnify each other on customary terms with regard to any information
provided by such party. Upon receiving any request under this Section 6 from any
Holder, Promistar agrees to send a copy thereof to any other person known to
Promistar to be entitled to registration rights under this Section 6, in each
case by promptly mailing the same, postage prepaid, to the address of record of
the persons entitled to receive such copies.


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         7. (a) Upon the occurrence of a Repurchase Event (as hereinafter
defined) that occurs prior to an Exercise Termination Event, (i) at the request
of the Holder, delivered prior to an Exercise Termination Event (or such later
period as provided in Section 9), Promistar shall repurchase the Option from the
Holder at a price (the "Option Repurchase Price") equal to the amount by which
(A) the Market/Offer Price (as defined below) exceeds (B) the Option Price,
multiplied by the number of shares for which this Option may then be exercised
and (ii) at the request of the owner of Option Shares from time to time (the
"Owner"), delivered prior to the occurrence of an Exercise Termination Event (or
such later period as provided in Section 9), Promistar shall repurchase such
number of the Option Shares from the Owner as the Owner shall designate at a
price (the "Option Share Repurchase Price") equal to the Market/Offer Price
multiplied by the number of Option Shares so designated. The term "Repurchase
Event" shall occur if (i) any person other than FNB or any of its Subsidiaries
shall have acquired beneficial ownership, or the right to acquire beneficial
ownership, or any "group" (as such term is defined under the 1934 Act) shall
have been formed which beneficially owns or has the right to acquire beneficial
ownership of 50% or more of the then-outstanding Common Shares, or (ii) any of
the transactions described in Section 8(a)(i), 8(a)(ii), or 8(a)(iii) hereof
shall be consummated. The term "Market/Offer Price" shall mean the highest of
(i) the price per share of Common Shares at which a tender or exchange offer
therefor has been made, (ii) the price per share of Common Shares to be paid by
any third party pursuant to an agreement with Promistar, (iii) the highest
closing price for Common Shares within the three-month period immediately
preceding the date the Holder gives notice of the required repurchase of this
Option or the Owner gives notice of the required repurchase of Option Shares, as
the case may be, or (iv) in the event of a sale of all or substantially all of
Promistar's assets or deposits, the sum of the net price paid in such sale for
such assets or deposits, the sum of the net price paid in such sale for such
assets or deposits and the current market value of the remaining net assets of
Promistar as determined by a nationally recognized investment banking firm
selected by the Holder or the Owner, as the case may be, divided by the number
of Common Shares of Promistar outstanding at the time of such sale. In
determining the Market/Offer Price, the value of consideration other than cash
shall be determined by a nationally recognized investment banking firm selected
by the Holder or Owner, as the case may be.

              (b) The Holder and the Owner, as the case may be, may exercise its
right to require Promistar to repurchase the Option and any Option Shares
pursuant to this Section 7 by surrendering for such purpose to Promistar, at its
principal office, a copy of this Agreement or certificates for Option Shares, as
applicable, accompanied by a written notice or notices stating that the Holder
or the Owner, as the case may be, elects to require Promistar to repurchase this
Option and/or the Option Shares in accordance with the provisions of this
Section 7. As promptly as practicable, and in any event within ten business days
after the surrender of the Option and/or certificates representing Option Shares
and the receipt of such notice or notices relating thereto, Promistar shall
deliver or cause to be delivered to the Holder the Option Repurchase Price
and/or to the Owner the Option Share Repurchase Price therefor or the portion
thereof that Promistar is not then prohibited under applicable law and
regulation from so delivering.

              (c) To the extent that Promistar is prohibited under applicable
law or regulation, or as a consequence of administrative policy, from
repurchasing the Option and/or the Option Shares in full, Promistar shall
immediately so notify the Holder and/or the Owner and thereafter deliver or
cause to be delivered, from time to time, to the Holder and/or the Owner, as
appropriate, the portion of the Option Repurchase Price and the Option Share
Repurchase Price, respectively, that it is no longer prohibited from delivering,
within ten business days after the date on which Promistar is no longer so
prohibited; provided, however, that if Promistar at any time after delivery of a
notice of repurchase pursuant to


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paragraph (b) of this Section 7 is prohibited under applicable law or
regulation, or as a consequence of administrative policy, from delivery to the
Holder and/or the Owner, as appropriate, the Option Repurchase Price and the
Option Share Repurchase Price, respectively, in full (and Promistar hereby
undertakes to use its best efforts to obtain all required regulatory and legal
approvals and to file any required notices as promptly as practicable in order
to accomplish such repurchase), the Holder or Owner may revoke its notice of
repurchase of the Option or the Option Shares whether in whole or to the extent
of the prohibition, whereupon, in the latter case, Promistar shall promptly (i)
deliver to the Holder and/or the Owner, as appropriate, that portion of the
Option Purchase Price or the Option Share Repurchase Price that Promistar is not
prohibited from delivering, and (ii) deliver, as appropriate, either (A) to the
Holder, a new Agreement evidencing the right of the Holder to purchase that
number of Common Shares obtained by multiplying the number of Common Shares for
which the surrendered Agreement was exercisable at the time of delivery of the
notice of repurchase by a fraction, the numerator of which is the Option
Repurchase Price less the portion thereof theretofore delivered to the Holder
and the denominator of which is the Option Repurchase Price, or (B) to the
Owner, a certificate for the Option Shares it is then so prohibited from
repurchasing.

         8. (a) In the event that, prior to an Exercise Termination Event,
Promistar shall enter into an agreement (i) to consolidate with or merge into
any person, other than FNB or a FNB Subsidiary, and shall not be the continuing
or surviving corporation of such consolidation or merger, (ii) to permit any
person, other than FNB or a FNB Subsidiary, to merge into Promistar and
Promistar shall be the continuing or surviving corporation, but, in connection
with such merger, the then outstanding Common Shares shall be changed into or
exchanged for stock or other securities of any other person or cash or any other
property or the then outstanding Common Shares shall after such merger represent
less than 50% of the outstanding shares and share equivalents of the merged
company, or (iii) to sell or otherwise transfer all or substantially all of its
or any Significant Subsidiary's assets or deposits to any person, other than FNB
or a FNB Subsidiary, then, and in each such case, the agreement governing such
transaction shall make proper provision so that the Option shall, upon the
consummation of any such transaction and upon the terms and conditions set forth
herein, be converted into, or exchanged for, an option (the "Substitute
Option"), at the election of the Holder, of either (x) the Acquiring Corporation
(as hereinafter defined) or (y) any person that controls the Acquiring
Corporation.

            (b) The following terms have the meanings indicated:

                (i)    "Acquiring Corporation" shall mean (i) the continuing or
surviving corporation of a consolidation or merger with Promistar (if other than
Promistar), (ii) Promistar in a merger in which Promistar is the continuing or
surviving person, and (iii) the transferee of all or substantially all of
Promistar's assets or deposits (or the assets or deposits of a Significant
Subsidiary of Promistar).

                (ii)   "Substitute Common Shares" shall mean the common shares
issued by the issuer of the Substitute Option upon exercise of the Substitute
Option.

                (iii)  "Assigned Value" shall mean the Market/Offer Price, as
defined in Section 7.

                (iv)   "Average Price" shall mean the average closing price of
the Substitute Common Share for the one year immediately preceding the
consolidation, merger or sale in question, but in no event higher than the
closing price of the substitute Common Shares on the day preceding such
consolidation, merger or sale; provided that if Promistar is the issuer of the
Substitute Option, the Average Price shall be computed with respect to common
shares issued by the person merging into Promistar or by any company which
controls or is controlled by such person, as the Holder may elect.


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              (c) The Substitute Option shall have the same terms as the Option,
provided, that if the terms of the Substitute Option cannot, for legal reasons,
be the same as the Option, such terms shall be as similar as possible and in no
event less advantageous to the Holder. The issuer of the Substitute Option shall
also enter into an agreement with the then Holder or Holders of the Substitute
Option in substantially the same form as this Agreement, which agreement shall
be applicable to the Substitute Option.

              (d) The Substitute Option shall be exercisable for such number of
Substitute Common Shares as is equal to the Assigned Value multiplied by the
number of Common Shares for which the Option is then exercisable, divided by the
Average Price. The exercise price of the Substitute Option per Substitute Common
Share shall then be equal to the Option Price multiplied by a fraction, the
numerator of which shall be the number of Common Shares for which the Option is
then exercisable and the denominator of which shall be the number of Substitute
Common Shares for which the Substitute Option is exercisable.

              (e) In no event, pursuant to any of the foregoing paragraphs,
shall the Substitute Option be exercisable for a number of shares which together
with shares of the Acquiring Corporation then beneficially owned by FNB,
constitutes more than 19.9% of the shares of Substitute Common Shares
outstanding prior to exercise of the Substitute Option.

         9.   The periods for exercise of certain rights under Sections 2, 6,
and 7 shall be extended: (i) to the extent necessary to obtain all regulatory
approvals for the exercise of such rights (for so long as the Holder is using
commercially reasonable efforts to obtain such regulatory approvals), and for
the expiration of all statutory waiting periods; and (ii) to the extent
necessary to avoid liability under Section 16(b) of the 1934 Act by reason or
such exercise.

         10.  Promistar hereby represents and warrants to FNB as follows:

              (a) Promistar has full corporate power and authority to execute
and deliver this Agreement and to consummate the transactions contemplated
hereby. The execution and delivery this Agreement and the consummation of the
transactions contemplated hereby have been duly and validly authorized by the
Board of Directors of Promistar and no other corporate proceedings on the part
of Promistar are necessary to authorize this Agreement or to consummate the
transactions so contemplated. This Agreement has been duly and validly executed
and delivered by Promistar.

              (b) Promistar has taken all necessary corporate action to
authorize and reserve and to permit it to issue, and at all times from the date
hereof through the termination of this Agreement in accordance with its terms
will have reserved for issuance upon the exercise of the Option, that number of
Common Shares equal to the maximum number of Common Shares at any time and from
time to time issuable hereunder, and all such shares, upon issuance pursuant
thereto, will be duly authorized, validly issued, fully paid, nonassessable.

         11.  Neither of the parties hereto may assign any of its rights or
obligations under this Agreement or the Option created hereunder to any other
person, without the express written consent of the other party, except that in
the event a Subsequent Triggering Event shall have occurred prior to an Exercise
Termination Event, FNB, subject to the express provisions hereof, may assign in
whole or in part its rights and obligations hereunder following such Subsequent
Triggering Event; provided, however that


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<PAGE>   10


until the date 30 days following the date on which the Federal Reserve Board has
approved applications by FNB to acquire the Common Shares subject to the Option,
FNB may not assign its rights under the Option except in (i) a widely dispersed
public distribution, (ii) a private placement in which no one party acquires the
right to purchase in excess of 2% of the voting shares of issuer, (iii) an
assignment to a single party (i.e., a broker or investment banker) for the
purpose of conducting a widely disbursed public distribution on FNB's behalf, or
(iv) any other manner approved by the Federal Reserve Board.

         12.  Each of FNB and Promistar will use its best efforts to make all
filings with, and to obtain consents of, all third parties and governmental
authorities necessary to the consummation of the transactions contemplated by
this Agreement, including without limitation applying to the Federal Reserve
Board under the Bank Holding Company Act for approval to acquire the shares
issuable hereunder, but FNB shall not be obligated to apply to state banking
authorities for approval to acquire the Common Shares issuable hereunder until
such time, if ever, as it deems appropriate to do so.

         13.  The parties hereto acknowledge that damages would be an inadequate
remedy for a breach of this Agreement by either party hereto and that the
obligations of the parties hereto shall be enforceable by either party hereto
through injunctive or other equitable relief.

         14.  If any term, provision, covenant or restriction contained in this
Agreement is held by a court or a federal or state regulatory agency of
competent jurisdiction to be invalid, void or unenforceable, the remainder of
the terms, provisions and covenants and restrictions contained in this Agreement
shall remain in full force and effect, and shall in no way be affected, impaired
or invalidated. If for any reason such court or regulatory agency determines
that the Holder is not permitted to acquire, or Promistar is not permitted to
repurchase pursuant to Section 7, the full number of Common Shares provided in
Section 1(a) hereof (as adjusted pursuant to Section 5 hereof), it is the
express intention of Promistar to allow the Holder to acquire or to require
Promistar to repurchase such lesser number of shares as may be permissible,
without any amendment or modification hereof.

         15.  All notices, requests, claims, demands and other communications
hereunder shall be deemed to have been duly given when delivered in person, by
fax, telecopy, or by registered or certified mail (postage prepaid, return
receipt requested) at the respective addresses of the parties set forth in the
Merger Agreement.

         16.  This Agreement shall be governed by and construed in accordance
with the laws of the State of Florida, regardless of the laws that might
otherwise govern under applicable principles of conflicts of laws thereof.

         17.  This Agreement may be executed in two or more counterparts, each
of which shall be deemed to be an original, but all of which shall constitute
one and the same agreement.

         18.  Except as otherwise expressly provided herein, each of the parties
hereto shall bear and pay all costs and expenses incurred by it or on its behalf
in connection with the transactions contemplated hereunder, including fees and
expenses of its own financial consultants, investment bankers, accountants and
counsel.

         19.  Except as otherwise expressly provided herein or in the Merger
Agreement, this Agreement contains the entire agreement between the parties with
respect to the transactions contemplated hereunder and supersedes all prior
arrangements or understandings with respect thereof, written or oral. The terms


                                       10


<PAGE>   11

and conditions of this Agreement shall inure to the benefit of and be binding
upon the parties hereto and their respective successors and permitted assignees.
Nothing in this Agreement, expressed or implied, is intended to confer upon any
party, other than the parties hereto, and their respective successors except as
assignees, any rights, remedies, obligations or liabilities under or by reason
of this Agreement, except as expressly provided herein.

         20.  Capitalized terms used in this Agreement and not defined herein
shall have the meanings assigned thereto in the Merger Agreement.


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<PAGE>   12


         IN WITNESS WHEREOF, each of the parties hereto has caused this
Agreement to be executed on its behalf by its officer thereunto duly authorized,
all as of the date first above written.


                                      F.N.B. CORPORATION


                                  By: /s/ Gary L. Tice
                                      ------------------------------------------
                                      Gary L. Tice
                                      President and Chief Executive Officer


                                      PROMISTAR FINANCIAL CORPORATION


                                  By: /s/ John H. Anderson
                                      ------------------------------------------
                                      John H. Anderson
                                      Chairman and Chief Executive Officer


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