<PAGE>
                                                                     EXHIBIT 4.3


                          FIRST SUPPLEMENTAL INDENTURE

      FIRST SUPPLEMENTAL INDENTURE dated as of March 25, 2003 (this
"SUPPLEMENTAL INDENTURE"), among Comcast Corporation, a Delaware corporation
(excluding its Subsidiaries, the "COMPANY" or "COMCAST"), Comcast Cable
Holdings, LLC, a Delaware limited liability company, (excluding its
Subsidiaries, "COMCAST CABLE HOLDINGS"), Comcast Cable Communications Holdings,
Inc., a Delaware corporation (excluding its Subsidiaries, "COMCAST CABLE
COMMUNICATIONS HOLDINGS"), Comcast Cable Communications, Inc., a Delaware
corporation (excluding its Subsidiaries, "COMCAST CABLE"), Comcast MO Group,
Inc., a Delaware corporation (excluding its Subsidiaries, "COMCAST MO GROUP" and
collectively with Comcast Cable Holdings, Comcast Cable Communications Holdings
and Comcast Cable, each an "ORIGINAL GUARANTOR"), Comcast MO of Delaware, Inc.,
a Delaware corporation (excluding its Subsidiaries, "CONTINENTAL") and The Bank
of New York, a New York banking corporation, as trustee (the "TRUSTEE").

      WHEREAS, the Company is the issuer under the Indenture, dated as of
January 7, 2003 among the Company, the Trustee and the Original Guarantors (the
"ORIGINAL INDENTURE" and together with this Supplemental Indenture, the
"INDENTURE"), pursuant to which the Company issued, and the Trustee
authenticated and delivered 5.85% Notes Due 2010, 6.50% Notes Due 2015, 5.50%
Notes Due 2011, and 7.05% Notes Due 2033 which are, as of the date hereof,
outstanding (the "OUTSTANDING SECURITIES") and pursuant to which the Company may
issue Securities in the future (the "NEW SECURITIES," and together with the
Outstanding Securities, the "SECURITIES");

      WHEREAS, each of Comcast Cable Communications Holdings, Comcast Cable,
Comcast MO Group, Comcast Cable Holdings and Continental is a Wholly-Owned
Subsidiary of Comcast;

      WHEREAS, the Company is the obligor with respect to the Securities;

      WHEREAS, the Original Guarantors irrevocably, fully and unconditionally
guaranteed, jointly and severally, on an unsecured basis, the full and punctual
payment (whether at maturity, upon redemption, or otherwise) of the principal of
and interest on, and all other amounts payable under, each Security, and the
full and punctual payment of all other amounts payable by the Company under the
Indenture;

      WHEREAS, Continental desires to irrevocably, fully and unconditionally
guarantee, jointly and severally with the Original Guarantors, on an unsecured
basis, the full and punctual payment (whether at maturity, upon redemption, or
otherwise) of the principal of and interest on, and all other amounts payable
<PAGE>
under, each Security, and the full and punctual payment of all other amounts
payable by the Company under the Indenture; and

      WHEREAS, the Company, the Original Guarantors and Continental have
requested that the Trustee execute and deliver this Supplemental Indenture and
all requirements necessary to make this Supplemental Indenture a valid
instrument in accordance with its terms and to make the guarantees provided
under the Indenture the valid obligations of Continental, and the execution and
delivery of this Supplemental Indenture have been duly authorized in all
respects.

      NOW, THEREFORE, in consideration of the mutual promises contained herein
and for other good and valuable consideration, the receipt and sufficiency of
which are hereby acknowledged, the Company, each Original Guarantor, Continental
and the Trustee hereby agree as follows for the equal and ratable benefit of the
holders of the Securities:

                                    ARTICLE 1
                                   DEFINITIONS

      Section 1.01. Certain Terms Defined. The Indenture is hereby amended as
follows:

      "CABLE GUARANTOR" means each of Comcast Cable Holdings, Comcast Cable
Communications Holdings, Comcast Cable, Comcast MO Group, and Continental.

                                    ARTICLE 2
                                   GUARANTEES

      Section 2.01. Guarantee. Continental hereby agrees to be bound by all
obligations of a Cable Guarantor as set forth under the Indenture including, but
not limited to irrevocably, fully and unconditionally guaranteeing, jointly and
severally, on an unsecured basis, the full and punctual payment (whether at
maturity, upon redemption, or otherwise) of the principal of and interest on,
and all other amounts payable under, each Security, and the full and punctual
payment of all other amounts payable by the Company under the Indenture, upon
the terms and subject to the conditions of the Indenture.

      The following Section 13.10 is hereby added to Article 13 of the Original
Indenture:

      "Section 13.10. Additional Cable Guarantors. If, from time to time, a
Comcast Subsidiary desires to be added as a Cable Guarantor under the Indenture


                                       2
<PAGE>
and such Subsidiary agrees to assume all the obligations of a Cable Guarantor
under the Indenture including without limitation, the obligations specified
under this Article 13, such Subsidiary may be added to the definition of "Cable
Guarantor" under the Indenture by entering into a written agreement with the
Company and the Trustee in the form attached as Exhibit A. Execution of such
written agreement by a Subsidiary evidences the Cable Guarantee of such
Subsidiary and constitutes due delivery of the Cable Guarantee as set forth in
this Supplemental Indenture on behalf of the Subsidiary with respect to the
Outstanding Securities. The execution of such written agreement evidences the
Cable Guarantee of such Subsidiary with respect to any New Security, whether or
not the person signing as an officer of the Subsidiary still holds that office
at the time of authentication of such New Security. The delivery of any New
Security by the Trustee after authentication constitutes due delivery of the
Cable Guarantees as set forth in this Supplemental Indenture on behalf of the
Subsidiary."

                                    ARTICLE 3
                                  MISCELLANEOUS

      Section 3.01. Date and Time of Effectiveness. This Supplemental Indenture
shall become a legally effective and binding instrument at and as of the date
hereof.

      Section 3.02. Supplemental Indenture Incorporated Into Indenture. The
terms and conditions of this Supplemental Indenture shall be deemed to be part
of the Indenture for all purposes relating to the Securities. The Original
Indenture is hereby incorporated by reference herein and the Original Indenture,
as supplemented by this Supplemental Indenture, is in all respects adopted,
ratified and confirmed.

      Section 3.03. Outstanding Securities Deemed Conformed. As of the date
hereof, the provisions of the Outstanding Securities shall be deemed o be
conformed, without the necessity for any reissuance or exchange of such
Outstanding Security or any other action on the party of the holders of
Outstanding Securities, the Company or the Trustee, so as to reflect this
Supplemental Indenture.

      Section 3.04. Separability. In case any provision in this Supplemental
Indenture, or in the Indenture, shall be invalid, illegal or unenforceable, the
validity, legality and enforceability of the remaining provisions shall not in
any way be affected or impaired thereby, it being intended that all of the
provisions hereof shall be enforceable to the full extent permitted by law.

      Section 3.05. Benefits of Supplemental Indenture. Nothing in this
Supplemental Indenture, expressed or implied, shall give or be construed to give


                                       3
<PAGE>
to any person, firm or corporation, other than the parties hereto and the
holders of Securities, any benefit or any legal or equitable right, remedy or
claim under this Supplemental Indenture or the Indenture.

      Section 3.06. Successors. Subject to Section 13.09 of the Original
Indenture dated as of January 7, 2003, all agreements of the Company, the
Guarantors and the Trustee in this Supplemental Indenture and in the Indenture
shall bind their respective successors.

      Section 3.07. New York Law to Govern. This Supplemental Indenture shall be
deemed to be a contract under the laws of the State of New York, and for all
purposes shall be construed in accordance with the laws of such State.

      Section 3.08. Counterparts. This Supplemental Indenture may be executed in
any number of counterparts, each of which shall be an original; but such
counterparts shall together constitute but one and the same instrument.

      Section 3.09. Effect Of Headings. The Article and Section headings herein
are for convenience only and shall not affect the construction hereof.

      Section 3.10 Trustee. The Trustee makes no representations as to the
validity or sufficiency of this Supplemental Indenture. The statements and
recitals herein are deemed to be those of the Company and the Guarantor and not
of the Trustee.



                                       4
<PAGE>
      IN WITNESS WHEREOF, each the parties have caused this Supplemental
Indenture to be duly executed, and its corporate seal to be hereunto affixed and
attested, all as of the first date written above.

                                    COMCAST CORPORATION


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

[CORPORATE SEAL]

Attest:

By:
    -------------------------------


                                    COMCAST CABLE COMMUNICATIONS HOLDINGS, INC.


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

[CORPORATE SEAL]

Attest:

By:
    -------------------------------


                                    COMCAST CABLE COMMUNICATIONS, INC.


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

[CORPORATE SEAL]

Attest:

By:
    -------------------------------
<PAGE>
                                    COMCAST MO GROUP, INC.


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

[CORPORATE SEAL]

Attest:

By:
    -------------------------------


                                    COMCAST CABLE HOLDINGS, LLC


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

[CORPORATE SEAL]

Attest:

By:
    -------------------------------


                                    COMCAST MO OF DELAWARE, INC.


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

[CORPORATE SEAL]

Attest:

By:
    -------------------------------
<PAGE>
                                    THE BANK OF NEW YORK, as Trustee


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

[CORPORATE SEAL]

Attest:

By:
    -------------------------------
<PAGE>
                                                                       Exhibit A

                         Additional Guarantor Agreement

      This Additional Guarantor Agreement (the "AGREEMENT") is entered into as
of ________, 200_ among Comcast Corporation, a Pennsylvania corporation (the
"COMPANY" or "COMCAST"), [________________ ] (the "NEW CABLE GUARANTOR"), and
The Bank of New York, a New York banking corporation, as trustee (the
"TRUSTEE").

      WHEREAS, the Company is the issuer under the Indenture, dated as of
January 7, 2003 and a First Supplemental Indenture dated as of March 25, 2003
(collectively, the "INDENTURE"), pursuant to which the Company issued, and the
Trustee authenticated and delivered, certain Securities (as defined below) which
are, as of the date hereof, outstanding (the "OUTSTANDING SECURITIES") and
pursuant to which the Company may issue Securities in the future (the "NEW
SECURITIES," and together with the Outstanding Securities, the "SECURITIES")
which Securities have been guaranteed by Comcast Cable Holdings, LLC, a Delaware
limited liability company, Comcast Cable Communications, Inc., a Delaware
corporation, Comcast Cable Communications Holdings, Inc., a Delaware
corporation, Comcast MO Group, Inc., a Delaware corporation, and Comcast MO of
Delaware, Inc., a Delaware corporation;

      WHEREAS, pursuant to Section 13.10 of the Indenture, the New Cable
Guarantor may become a Cable Guarantor under the Indenture by entering into this
Agreement among the New Cable Guarantor, the Company and the Trustee; and

      WHEREAS, the New Cable Guarantor wishes to enter into this Agreement to
make the guarantees as provided under the Indenture a valid obligation of the
New Cable Guarantor, and the execution and delivery of this Agreement have been
duly authorized in all respects;

      NOW, THEREFORE, in consideration of the mutual promises contained herein
and for other good and valuable consideration, the receipt and sufficiency of
which are hereby acknowledged, the Company, the New Cable Guarantor, and the
Trustee hereby agree as follows for the equal and ratable benefit of the holders
of the Securities:

      1. The parties hereby agree to the addition of the New Cable Guarantor to
the definition of "CABLE GUARANTOR" under the Indenture and the New Guarantor
agrees to be bound by all obligations of a Cable Guarantor as set forth under
the Indenture including, but not limited to irrevocably, fully and
unconditionally guaranteeing, jointly and severally, on an unsecured basis, the
full and punctual payment (whether at maturity, upon redemption, or otherwise)
of the
<PAGE>
principal of and interest on, and all other amounts payable under, each
Security, and the full and punctual payment of all other amounts payable by the
Company under the Indenture, upon the terms and subject to the conditions of the
Indenture.

      2. This Agreement shall be deemed to be a contract under the laws of the
State of New York, and for all purposes shall be construed in accordance with
the laws of such State.

      3. This Agreement may be executed in any number of counterparts, each of
which shall be an original; but such counterparts shall together constitute but
one and the same instrument.

                            [Signature page follows]
<PAGE>
      IN WITNESS WHEREOF, each the parties have caused this Agreement to be duly
executed, and its corporate seal to be hereunto affixed and attested, all as of
the first date written above.

                                    COMCAST CORPORATION


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

[CORPORATE SEAL]

Attest:

By:
    -------------------------------


                                    [                      ]
                                     ----------------------


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

[CORPORATE SEAL]

Attest:

By:
    -------------------------------


                                    THE BANK OF NEW YORK, as Trustee


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

[CORPORATE SEAL]

Attest:

By:
    -------------------------------