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                                                                    EXHIBIT 4(n)
 
                                                                [EXECUTION COPY]
 
                                AMENDMENT NO. 2
 
     AMENDMENT NO. 2 dated as of December 6, 1996, among FIRST MISSISSIPPI
CORPORATION, a corporation duly organized and validly existing under the laws of
the State of Mississippi (the "Old Company"); CHEMFIRST INC., a corporation duly
organized and validly existing under the laws of the State of Mississippi (the
"New Company"); each of the banks that is a signatory hereto (individually, a
"Bank" and, collectively, the "Banks"); and THE CHASE MANHATTAN BANK, a New York
State banking corporation and successor by merger to The Chase Manhattan Bank,
N.A., as agent for the Banks (in such capacity, together with its successors in
such capacity, the "Agent").
 
     The Old Company, the Lenders and the Agent are parties to a Credit
Agreement dated as of February 9, 1993 (as modified and supplemented and in
effect on the date hereof, the "Credit Agreement"). Pursuant to Section 6.11 of
an Agreement and Plan of Merger and Reorganization dated as of August 27, 1996
by and among Mississippi Chemical Corporation, Miss Sub, Inc. and the Old
Company (the "Reorganization Agreement"), the Old Company proposes to contribute
to the New Company all of its assets (other than those relating to the Retained
Business under and as defined in the Reorganization Agreement) and has requested
the consent of the Banks and the Agent thereto (and the consent of the Banks and
the Agent to the assignment by the Old Company to the New Company of all of the
rights of the Old Company, and the assumption by the New Company of all of the
obligations of the Old Company, under the Credit Agreement). The Lenders are
willing to consent to such assignment and assumption and to amend the Credit
Agreement in certain respects as hereinafter provided and, accordingly, the
parties hereto hereby agree as follows:
 
     Section 1. Definitions. Except as otherwise defined in this Amendment No.
2, terms defined in the Credit Agreement are used herein as defined therein.
 
     Section 2. Amendments Effective Immediately. Subject to the execution
hereof by the Old Company, the New Company, Banks constituting the "Majority
Banks" under the Credit Agreement and the Agent, Section 8.13 of the Credit
Agreement is hereby amended in its entirety to read as follows:
 
          "8.13 Cash Flow Ratio. The ratio of Cash Flow for any period to the
     sum of (i) interest expense for such period, (ii) the aggregate amount of
     Payments of Indebtedness for such period and (iii) the aggregate amount of
     Dividend Payments made during such period, shall not be less than 1.2 to 1
     for any period of four consecutive fiscal quarters of the Company, provided
     that such ratio shall be not less than .4 to 1 for the period of four
     consecutive fiscal quarters of the Company ending December 31, 1996."
 
     Section 3. Assignment and Assumption; Amendments Effective upon
Spin-Off. Without limiting the generality of the provisions of Section 6.11 of
the Reorganization Agreement and the Distribution Agreement therein referred to,
effective immediately prior to the Time of Distribution under and as defined in
the Distribution Agreement, the Old Company assigns all of its right, title and
interest in, to and under the Credit Agreement and the Notes to the New Company,
and the New Company assumes all of the obligations of the Old Company in, to and
under the Credit Agreement and the Notes. Subject to the satisfaction of the
conditions precedent set forth in Section 5 hereof, (i) each of the Banks and
the Agent hereby consents, anything to the contrary in the Credit Agreement
notwithstanding (including, without limitation, Sections 8.03 and 8.05 thereof),
to the contribution by the Old Company to the New Company, as contemplated by
Section 6.11 of the Reorganization Agreement and the Distribution Agreement, of
all of the assets of the Old Company (other than the Retained Business) and to
the assignment and assumption of the rights and obligations of the Old Company
under the Credit Agreement and the Notes to the New Company pursuant to the
preceding sentence and (ii) each of the parties hereto hereby agrees that,
effective upon the assignment and assumption pursuant to immediately preceding
sentence, the definition of "the Company" under and for all purposes of the
Credit Agreement is hereby amended to refer to the New Company (and that the Old
Company shall be automatically released from all of its obligations under the
Credit Agreement and the
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Notes). In that connection, each of the Banks is hereby authorized by the New
Company to make an appropriate notation on the Note held by such Bank to the
effect that the New Company has assumed all of the obligations of the Old
Company under such Note.
 
     Upon the consummation of the Spin-Off, and subject to the satisfaction of
the conditions precedent set forth in Section 5 hereof, the Credit Agreement
shall be amended as follows (it being understood that the amendments set forth
below shall supersede the amendment set forth in Section 2 hereof):
 
     A. Section 1.01 of the Credit Agreement shall be amended by adding the
following new definitions (to the extent not already included in said Section
1.01) and inserting the same in the appropriate alphabetical locations and
amending in their entirety the following definitions (to the extent already
included in said Section 1.01), as follows:
 
          "Cash Flow" shall mean, for any period, the sum of the following for
     the Company and its Consolidated Subsidiaries for such period, all
     determined on a consolidated basis in accordance with GAAP (without
     duplication): (i) Net Income (or losses); plus (ii) taxes; plus (iii)
     interest expense; plus (iv) depreciation, depletion and amortization
     expenses and similar non-cash charges; plus (v) 75% of the net proceeds
     from the issuance to any non-Affiliate by the Company or any Consolidated
     Subsidiary of the Company of any equity securities; plus (vi) 50% of the
     net proceeds from any asset disposition announced and completed after the
     date hereof (including 50% of the net proceeds received by the Company in
     connection with the Spin-Off up to but not exceeding an amount equal to
     $75,000,000); and minus (vii) Capital Expenditures.
 
          "Consolidated Net Worth" shall mean, as at any date of determination,
     the stockholders' equity of the Company and its Consolidated Subsidiaries
     on a consolidated basis, minus any write-up in the book value of assets
     resulting from a revaluation thereof subsequent to June 30, 1996 in excess
     of the respective amounts shown on the pro forma balance sheet as at said
     date referred to in Section 7.02(ii) hereof.
 
          "Consolidated Tangible Net Worth" shall mean, as at any date of
     determination, the sum of the following for the Company and its
     Consolidated Subsidiaries on a consolidated basis: (1) the amount of issued
     and outstanding share capital (less the cost of treasury shares if not
     deducted in calculating the amount of share capital), plus (2) the amount
     of surplus and retained earnings (or in the case of a deficit, minus the
     amount of such deficit), minus (3) the sum of the following (without
     duplication of deductions in respect of items already deducted in arriving
     at surplus and retained earnings): minority interests in Subsidiaries; all
     reserves (except contingency reserves not allocated to specific purposes
     and not deducted from assets) which are properly treated as appropriations
     of surplus or retained earnings; the book value of all assets which would
     be treated as intangibles under GAAP, including, without limitation,
     goodwill, trademarks, trade-names, patents and unamortized debt discount
     and expense, and any write-up in book value of assets resulting from a
     revaluation thereof subsequent to June 30, 1996 in excess of the respective
     amounts shown on the pro forma balance sheet as at said date referred to in
     Section 7.02(ii) hereof.
 
          "Distribution Agreement" shall mean an Agreement and Plan of
     Distribution between the Old Company and the New Company in the form of
     Annex 1 attached to the Reorganization Agreement.
 
          "Employee Benefits and Compensation Agreement" shall mean an Employee
     Benefits and Compensation Agreement between the Old Company and the New
     Company in the form of Exhibit B to the Distribution Agreement.
 
          "New Company" shall refer to ChemFirst Inc., the "Company" under this
     Agreement after the consummation of the Spin-Off contemplated by Amendment
     No. 2 hereto.
 
          "Old Company" shall refer to First Mississippi Corporation, the
     "Company" under this Agreement prior to the consummation of the Spin-Off
     contemplated by Amendment No. 2 hereto.
 
          "Relevant Financial Information" shall mean the financial statements
     of the Company and its Consolidated Subsidiaries for the then most recent
     complete fiscal period of the Company furnished to the Banks in accordance
     with Section 8.01(a), 8.01(b) or 8.01(c) hereof, provided that prior to the
 
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     delivery to the Banks of the first such financial statements after the
     Spin-Off, such term shall refer to the pro forma financial statements for
     the fiscal quarter ended September 30, 1996 referred to in Section
     7.02(iii) hereof.
 
          "Reorganization Agreement" shall have the meaning assigned to such
     term in Amendment No. 2 hereto.
 
          "SEC" shall mean the Securities and Exchange Commission, or any
     successor governmental entity.
 
          "Spin-Off" shall have the meaning assigned to such term in the
     Distribution Agreement.
 
          "Tax Disaffiliation Agreement" shall mean a Tax Disaffiliation
     Agreement between the Company and the New Company in the form of Exhibit A
     to the Distribution Agreement.
 
          "Tranche A Commitment Termination Date" shall mean, for any Bank,
     February 9, 1998.
 
     B. Section 1.03(c) of the Credit Agreement is hereby amended in its
entirety to read as follows:
 
          "(c) To enable the ready and consistent determination of compliance
     with the covenants set forth in Section 8 hereof, the Company will not
     change the last day of its fiscal year from June 30, or the last day of the
     first three fiscal quarters in each of its fiscal years from September 30,
     December 31 and March 31, respectively, provided that, effective upon the
     Spin-Off, the Company shall change the last day of its fiscal year to
     December 31 (resulting in a short fiscal year ending December 31, 1996) and
     will not thereafter change the last day of its fiscal year from December
     31, or the last day of the first three fiscal quarters in each of its
     fiscal years from March 31, June 30 and September 30 in each year."
 
     C. Section 2.04 of the Credit Agreement is hereby amended in its entirety
to read as follows:
 
          "2.04  Intentionally Left Blank. This Section 2.04 has been
     intentionally left blank."
 
     D. Section 7.02 of the Credit Agreement shall be amended in its entirety to
read as follows:
 
          "7.02  Financial Condition. The Company has delivered to the Banks
     prior to the Spin-Off the following financial statements:
 
             (i) the audited consolidated balance sheet and statements of
        operations, changes in stockholders' equity and cash flows of the
        Company and its Subsidiaries as of and for the fiscal year ended June
        30, 1996, reported upon by KPMG Peat Marwick LLP;
 
             (ii) the unaudited pro forma consolidated balance sheet and
        statement of operations of the Company and its Subsidiaries as of and
        for the fiscal year ended June 30, 1996, prepared under the assumption
        that the transactions contemplated by the Distribution Agreement had
        occurred on July 1, 1995, which statement of operations is set forth in
        the Joint Proxy Statement/Prospectus on Schedule 14A pursuant to which
        the registration of the shares of common stock of the New Company to be
        issued in connection with the Spin-Off is to be effected; and
 
             (iii) the unaudited pro forma consolidated balance sheet and
        statement of operations of the Company and its Subsidiaries for the
        fiscal quarter ended September 30, 1996, prepared under the assumption
        that the transactions contemplated by the Distribution Agreement had
        occurred on July 1, 1996, which balance sheet and statement of
        operations is set forth in said Joint Proxy Statement/Prospectus.
 
     The financial statements referred to in clauses (i) and (ii) above present
     fairly, in all material respects, the actual or pro forma (as the case may
     be) consolidated financial position and results of operations and (in the
     case of clause (i) cash flows of the Company and its Subsidiaries as of
     such dates and for such periods in accordance with generally accepted
     accounting principles and practices applied on a consistent basis, except
     (in the case of such pro forma consolidated financial statements) as
     required by the rules and regulations of the SEC (including Regulation S-X
     issued by the SEC). The pro forma financial
 
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     statements referred to in clause (iii) above present fairly, in all
     material respects, the pro forma consolidated financial position and
     results of operations of the Company and its Subsidiaries as of September
     30, 1996, and for the fiscal quarter ended on said date, and have been
     prepared on a basis consistent with the preparation of the audited
     financial statements as at June 30, 1996 referred to in clause (i) above.
     Neither the Company nor any of its Subsidiaries had on said dates any
     material contingent liabilities, liabilities for taxes, unusual forward or
     long-term commitments or unrealized or anticipated losses from any
     unfavorable commitments, except as referred to or reflected or provided for
     in the balance sheets as at said dates. Since September 30, 1996, there has
     been no material adverse change in the consolidated financial condition,
     operations, business or prospects taken as a whole of the Company and its
     Consolidated Subsidiaries from that set forth in said financial statements
     as at said date."
 
     E. Section 7.09 of the Credit Agreement is hereby amended by deleting the
reference therein to "June 30, 1982" and inserting "June 30, 1988" in lieu
thereof.
 
     F. Section 8.08 of the Credit Agreement shall be amended in its entirety to
read as follows:
 
          "8.08  Intentionally Left Blank. This Section 8.08 is intentionally
     left blank."
 
     G. Section 8.11 of the Credit Agreement shall be amended in its entirety to
read as follows:
 
          "8.11  Consolidated Tangible Net Worth. The Company shall not permit
     the Consolidated Tangible Net Worth to be less than $290,000,000 at any
     time; provided that the Company shall not permit the Consolidated Tangible
     Net Worth as at the first day of each fiscal year of the Company
     (commencing with the fiscal year starting January 1, 1998) and for each day
     in such fiscal year thereafter to be less than the sum of (i) $290,000,000
     or, if greater, the amount required hereunder for the immediately preceding
     fiscal year plus (ii) an amount (not less than zero) equal to 30% of the
     Net Income of the Company and its Consolidated Subsidiaries for the
     immediately preceding fiscal year."
 
     H. Section 8.13 of the Credit Agreement shall be amended in its entirety to
read as follows:
 
          "8.13  Cash Flow Ratio. The ratio of Cash Flow for any period to the
     sum of (i) interest expense for such period, (ii) the aggregate amount of
     Payments of Indebtedness for such period and (iii) the aggregate amount of
     Dividend Payments made during such period, shall not be less than 1.20 to
     1, for any period of four consecutive fiscal quarters of the Company ending
     on or after the Spin-Off, it being understood that, if all or any portion
     of the period for which the components of such ratio is being calculated
     shall occur prior to the Spin-Off, such components shall be calculated on a
     consolidated basis for the Old Company and its Subsidiaries."
 
     I. A new Section 8.16 of the Credit Agreement shall be added to the Credit
Agreement to read in its entirety as follows:
 
          "8.16  Modifications of Certain Documents. The Company will not,
     following the execution and delivery thereof, consent to any modification,
     supplement or waiver of any of the provisions of the Distribution
     Agreement, Tax Disaffiliation Agreement or the Employee Benefits and
     Compensation Agreement without the prior consent of the Majority Banks."
 
     J. Section 9.01(d) of the Credit Agreement shall be amended by deleting the
reference therein to "8.15" and inserting "8.16" in lieu thereof.
 
     K. Effective upon the Spin-Off, each of Schedules I, II and III to the
Credit Agreement shall be amended in their entireties to read as set forth in
Schedules I, II and III to this Amendment No. 2.
 
     Section 4. Representations and Warranties. The New Company hereby
represents and warrants to the Banks that, effective upon the Spin-Off:
 
          (a) Credit Agreement Representations. Each of the representations and
     warranties of "the Company" set forth in Sections 7.01 through 7.14 of the
     Credit Agreement (inclusive) are true and complete
 
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     on and as of the date of the Spin-Off giving effect to the provisions of
     Section 3 hereof (including that each reference in said Sections to "the
     Company" shall be deemed to be a reference to the New Company and the
     amendments to Section 7.02, and Schedules I, II and III, of the Credit
     Agreement) and as if each reference therein to "this Agreement" included
     reference to this Amendment No. 2 and to the Distribution Agreement, the
     Tax Disaffiliation Agreement and the Employee Benefits and Compensation
     Agreement.
 
          (b) Indebtedness and Cash. After giving effect to the Spin-Off, (i)
     neither the New Company nor any of its Subsidiaries will be obligated in
     respect of any Indebtedness (other than Indebtedness under the Credit
     Agreement or listed on Schedule III hereto) and (ii) the Old Company shall
     have contributed to the New Company, as contemplated by Section 6.14 of the
     Reorganization Agreement, excess proceeds of the "Financing" referred to
     therein in an aggregate amount at least equal to $50,000,000.
 
          (c) Disclosure. The New Company has heretofore delivered to each Bank
     a complete and correct copy of the Joint Proxy Statement/Prospectus on
     Schedule 14A pursuant to which the registration of the shares of common
     stock of the New Company to be issued in connection with the Spin-Off is to
     be effected. None of the reports, financial statements, certificates or
     other information (including said Registration Statement) furnished by or
     on behalf of the New Company to the Banks in connection with the
     negotiation of this Amendment No. 2 or delivered hereunder (as modified or
     supplemented by other information so furnished) contains any material
     misstatement of fact or omits to state any material fact necessary to make
     the statements therein, in the light of the circumstances under which they
     were made, not materially misleading; provided that, with respect to
     projected financial information, the New Company represents only that such
     information was prepared in good faith based upon assumptions believed to
     be reasonable at the time.
 
          (d) Consummation of Spin-Off. The transactions contemplated by the
     Distribution Agreement (including, without limitation, the transfer of the
     assets of the Old Company, other than the Retained Business under and as
     defined in the Reorganization Agreement, to the New Company as contemplated
     by Section 4 of the Distribution Agreement, the consummation of the
     Spin-Off and the execution and delivery of the Tax Disaffiliation Agreement
     and Employee Benefits and Compensation Agreement in the forms provided for
     in the Distribution Agreement) have been consummated substantially
     simultaneously and in all material respects in the manner provided for in
     the Reorganization Agreement and the Distribution Agreement.
 
     (e) No Default. No Default or Event of Default has occurred and is
continuing.
 
     Section 5. Conditions. The consent of the Lenders, and the amendments to
the Credit Agreement, set forth in Section 3 hereof shall become effective upon
the fulfillment of the following conditions precedent:
 
          A. Execution. This Amendment No. 2 shall have been duly executed and
     delivered by the Old Company, the New Company, each Bank and the Agent.
 
          B. Consummation of Spin-Off. The Agent shall have received evidence
     that the transactions contemplated by the Distribution Agreement
     (including, without limitation, the transfer of the assets of the Old
     Company, other than the Retained Business under and as defined in the
     Reorganization Agreement, to the New Company as contemplated by Section 4
     of the Distribution Agreement, the consummation of the Spin-Off and the
     execution and delivery of the Tax Disaffiliation Agreement and Employee
     Benefits and Compensation Agreement in the forms provided for in the
     Distribution Agreement) shall have been consummated substantially
     simultaneously and in all material respects in the manner provided for in
     the Reorganization Agreement and the Distribution Agreement, and the Agent
     shall have received a certificate of a senior financial officer of the New
     Company to such effect and to the effect that attached thereto are true and
     complete copies of the documents delivered in connection with the closing
     under the Distribution Agreement. In addition, the Agent shall have
     received copies of the legal opinions delivered to the New Company pursuant
     thereto, together with a letter from each Person delivering such opinion
     (or authorization within such opinion) authorizing reliance thereon by the
     Agent and the Banks.
 
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          C. Corporate Action. The Agent shall have received certified copies of
     the charter and by-laws of the New Company and all corporate action
     (including all shareholder action) taken by the New Company approving this
     Amendment No. 2 and the Credit Agreement as amended hereby, the Notes and
     any Letter of Credit applications and extensions of credit to the New
     Company hereunder (including, without limitation, a certificate setting
     forth the resolutions of the Board of Directors of the New Company adopted
     in respect of the transactions contemplated hereby).
 
          D. Incumbency. The Agent shall have received a certificate of the New
     Company in respect of each of the officers (i) who is authorized to sign on
     its behalf this Amendment No. 2 or any Letter of Credit application and
     (ii) who will, until replaced by another officer or officers duly
     authorized for that purpose, act as its representative for the purposes of
     signing documents and giving notices and other communications in connection
     with the Credit Agreement as amended hereby and the transactions
     contemplated by the Credit Agreement as amended hereby (and the Agent and
     each Bank may conclusively rely on such certificate until it receives
     notice in writing from the New Company to the contrary).
 
          E. Officer's Certificate. The Agent shall have received a certificate
     of a senior officer of the New Company that the representations and
     warranties set forth in Section 4 hereof are true and complete on and as of
     the date of the Spin-Off.
 
          G. Opinion of Counsel to the New Company. An opinion of Steve Chustz,
     General Counsel to the New Company, substantially in the form of Exhibit A
     hereto.
 
          H. Fees and Expenses. The Agent shall have received all fees and other
     amounts due and payable in connection with this Amendment No. 2, including,
     to the extent invoiced, reimbursement or payment of all out-of-pocket
     expenses required to be reimbursed or paid by the New Company under the
     Credit Agreement in connection herewith.
 
          I. Other Documents. Such other documents relating to the transactions
     contemplated hereby as the Agent or any Bank or Milbank, Tweed, Hadley &
     McCloy, special New York counsel to Chase, shall have reasonably requested
     in writing.
 
     Section 6. Miscellaneous. Except as herein provided, the Credit Agreement
shall remain unchanged and in full force and effect. This Amendment No. 2 may be
executed in any number of counterparts, all of which taken together shall
constitute one and the same amendatory instrument and any of the parties hereto
may execute this Amendment No. 2 by signing any such counterpart. This Amendment
No. 2 shall be governed by, and construed in accordance with, the law of the
State of New York.
 
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     IN WITNESS WHEREOF, the parties hereto have caused this Amendment No. 2 to
be duly executed and delivered as of the day and year first above written.
 
<TABLE>
<S>                                                    <C>
FIRST MISSISSIPPI CORPORATION                          CHEMFIRST INC.
 
By /s/ R. M. SUMMERFORD                                By /s/ R. M. SUMMERFORD
   --------------------------------------------------  --------------------------------------------------
   Title: Vice President                                  Title: Vice President
 
THE CHASE MANHATTAN BANK, successor by merger to The   BANK OF AMERICA ILLINOIS (as successor in interest to
  Chase Manhattan Bank, N.A., individually and as      Continental Bank, N.A.)
  Agent
 
By /s/ SCOTT S. WARD                                   By /s/ DEIRDRE B. DOYLE
   --------------------------------------------------  --------------------------------------------------
   Title: Vice President                                  Title: Vice President
</TABLE>
 
DEPOSIT GUARANTY NATIONAL BANK
 
By     /s/ E. ANTHONY THOMAS
   ---------------------------------
   Title: Senior Vice President
 
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