Document:

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                                                                  EXHIBIT 4.20

                       ASSIGNMENT AND ASSUMPTION AGREEMENT

                                       to

                   COMMON TRUST SECURITIES GUARANTEE AGREEMENT

                                     between

                         AMERICAN MUTUAL HOLDING COMPANY

                                       and

                           AMERUS LIFE HOLDINGS, INC.

                            Dated September 20, 2000

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                       ASSIGNMENT AND ASSUMPTION AGREEMENT

     This ASSIGNMENT AND ASSUMPTION AGREEMENT ("Agreement"), dated as of
September 20, 2000, is executed and delivered by AMERICAN MUTUAL HOLDING
COMPANY, an Iowa mutual insurance holding company ("AMHC") and AMERUS LIFE
HOLDINGS, INC., an Iowa insurance holding company ("AmerUs").

                                   WITNESSETH:

     WHEREAS, AmerUs Capital II (the "Trust") was established as a business
trust under the Delaware Business Trust Act pursuant to a Trust Agreement, dated
as of April 14, 1998 (the "Original Declaration"), between AmerUs and First
Union Trust Company, National Association, as Trustee, and a Certificate of
Trust executed and filed with the Secretary of State of the State of Delaware on
April 14, 1998, for the sole purpose of issuing and selling certain securities
representing undivided beneficial interests in the assets of the Trust and
investing the proceeds thereof in certain Debentures (as defined in the
Declaration) of AmerUs;

     WHEREAS, the Original Declaration was amended and restated, as of July 27,
1998 (the "Declaration"), by AmerUs, the trustees of the Trust, the
administrators of the Trust, and the holders of undivided beneficial interests
in the assets of the Trust to be issued from time to time, to provide for the
operation of the Trust and the issuance of Capital Securities and Common Trust
Securities;

     WHEREAS, the Guarantor executed a Common Trust Securities Guarantee
Agreement, dated July 27, 1998 (the "Common Trust Securities Guarantee
Agreement"), for the benefit of the Holders from time to time of the Common
Trust Securities of the Trust, whereby the Guarantor irrevocably and
unconditionally agreed, to the extent set forth in the Common Trust Securities
Guarantee Agreement, to pay to the Holders the Guarantee Payments and to make
certain other payments on the terms and conditions set forth in the Common Trust
Securities Guarantee Agreement;

     WHEREAS, AMHC has executed a Plan of Conversion, dated as of December 17,
1999, pursuant to which AMHC will convert from a mutual insurance holding
company into a stock holding company;

     WHEREAS, AMHC and the Guarantor have entered into an Agreement and Plan of
Merger, dated December 17, 1999, pursuant to which the Guarantor will merge with
and into AMHC, with AMHC as the surviving company (the "Merger") and AMHC will
thereafter change its name to AmerUs Group Co.;

     WHEREAS, AMHC desires to assume the Guarantor's obligations under the
Common Trust Securities Guarantee Agreement;

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     WHEREAS, this Assumption Agreement is being executed pursuant to Section
5.01 of the Common Trust Securities Guarantee Agreement; and

     WHEREAS, the Merger is permitted under Article Seven of the Indenture.

     NOW, THEREFORE, the parties agree as follows:

SECTION 1.  Assignment.  Upon the execution of this Agreement by the parties
            hereto, AmerUs does hereby assign and transfer all of its
            obligations under the Common Trust Securities Guarantee Agreement
            to AMHC, as its successor.

SECTION 2.  Assumption. Upon the execution of this Agreement by the
            parties hereto, AMHC does hereby absolutely and irrevocably
            accept the foregoing assignment and hereby assumes to be
            solely responsible for and to perform, as successor, AmerUs'
            obligations under the Common Trust Securities Guarantee
            Agreement.

SECTION 3.  Definition. For all purposes of this Agreement, except as
            otherwise herein expressly provided, the definitions, terms
            and expressions used herein shall have the same meanings as
            corresponding definitions, terms and expressions used in the
            Common Trust Securities Guarantee Agreement.

SECTION 4.  Effectiveness.  This Agreement shall become effective as of the
            Effective Time (as defined in the Agreement and Plan of Merger
            referred to above) of the Merger.

SECTION 5.  Binding Effect; Assigns.  This Agreement shall be binding upon,
            and shall enure to the benefit of, the parties thereto and their
            respective successors and assigns.

SECTION 6.  Counterparts.  This Agreement may be executed in two or more
            counterparts, each of which will be deemed to be an original,
            but all of which together will constitute one and the same
            agreement.

SECTION 7.  Governing Law.  This Agreement will be governed by and interpreted
            in accordance with the internal laws of the State of New York,
            without giving effect to principles of conflicts of laws.

            IN WITNESS WHEREOF, the parties hereto have executed and delivered
this Assumption Agreement as of the date set forth above.

                                        AMERICAN MUTUAL HOLDING COMPANY

                                        By: s/ Roger K. Brooks
                                            -----------------------------------
                                            Name:  Roger K. Brooks
                                            Title: Chairman, President and
                                                   Chief Executive Officer

                                        AMERUS LIFE HOLDINGS, INC.

                                       2

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                                        By: /s/ Roger K. Brooks
                                            -----------------------------------
                                            Name:  Roger K. Brooks
                                            Title: Chairman, President and
                                                   Chief Executive Officer

                                       3<PAGE>   1

                                                                    EXHIBIT 4.21

                                FIRST SUPPLEMENT

                                       to

                               PURCHASE CONTRACTS

                                     between

                         AMERICAN MUTUAL HOLDING COMPANY

                                       and

                                    HOLDERS,
                                  as specified

                            Dated September 20, 2000

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                          FIRST SUPPLEMENTAL AGREEMENT

     This First Supplemental Agreement (this "First Supplemental Agreement"),
dated as of September 20, 2000, is among AMERICAN MUTUAL HOLDING COMPANY, an
Iowa mutual insurance holding company ("AMHC"), as successor to AMERUS LIFE
HOLDINGS, INC., an Iowa insurance holding company (the "Company") and HOLDERS,
as specified on the Unit Certificates (the "Holders").

     WHEREAS, the Company and the Unit Agent entered into a Master Unit
Agreement, dated as of July 27, 1998 (the "Master Unit Agreement"), in which the
obligations of the Company under the Units are provided;

     WHEREAS, the Company has entered into a Purchase Contract with each Holder,
as evidenced by each Unit Certificate (the "Purchase Contracts"), which
obligates the Holder to purchase, and the Company to sell, Common Stock on the
Stock Purchase Date;

     WHEREAS, AMHC has executed a Plan of Conversion, dated as of December 17,
1999, pursuant to which AMHC will convert from a mutual insurance holding
company into a stock holding company;

     WHEREAS, the Company and AMHC have entered into an Agreement and Plan of
Merger, dated December 17, 1999, pursuant to which the Company will merge with
and into AMHC, with AMHC continuing as the surviving company (the "Merger") and
AMHC will thereafter change its name to AmerUs Group Co.; and

     WHEREAS, AMHC wishes to assume the obligations of the Company under the
Purchase Contracts, pursuant to the requirements of Article Nine of the Master
Unit Agreement, as authorized by Article Eight of the Master Unit Agreement and
without the consent of the Holders.

     NOW THEREFORE, in consideration of the foregoing, the parties agree as
follows:

Section 1.      Capitalized terms not otherwise defined herein shall have
                the meanings set forth in the Master Unit Agreement and the
                Purchase Contracts.

Section 2.      AMHC hereby agrees to assume the obligations of the
                Company, as its successor, under the Purchase Contracts and to
                be bound by the provisions of such \ agreements.

Section 3.      This First Supplemental Agreement shall become effective as
                of the Effective Time (as defined in the Agreement and Plan of
                Merger referred to above) of the Merger.

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Section 4.      This First Supplemental Agreement may be executed in two or
                more counterparts, each of which will be deemed to be an
                original, but all of which together will constitute one and
                the same agreement.

Section 5.      This Supplemental Agreement will be governed by and
                interpreted in accordance with the internal laws of the State
                of New York, without giving effect to principles of conflicts
                of laws.

                IN WITNESS WHEREOF, the parties hereto have executed and
delivered this Supplemental Agreement as of the date set forth above.

                                    AMERICAN MUTUAL HOLDING COMPANY

                                    By:  /s/ Roger K. Brooks
                                         --------------------------------------
                                    Name:  Roger K. Brooks
                                    Title: Chairman, President and
                                           Chief Executive Officer

                                    HOLDERS

                                    By:  First Union National Bank,
                                          as Attorney-in-Fact of such Holders

                                    By:  /s/ Shawn K. Bednasek
                                    -------------------------------------------
                                    Name:  Shawn K. Bednasek
                                    Title: Vice President

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