<PAGE>

                                                                     EXHIBIT 28A

                        MONTHLY SERVICER'S CERTIFICATE
                             First USA Bank, N.A.
        _______________________________________________________________

                         First Chicago Master Trust II
                               January 12, 2000
        _______________________________________________________________

The undersigned, duly authorized representatives of First USA Bank, National
Association ("First USA"), as Servicer pursuant to the Pooling and Servicing
Agreement dated as of June 1, 1990, as amended and restated as of
September 1, 1999, as amended and supplemented, (the "Pooling and Servicing
Agreement") by and between First USA, as Seller and Servicer and Norwest Bank
Minnesota, National Association, as Trustee, does hereby certify as follows:

<TABLE>
     <S>                                                                                       <C>
     1.   Capitalized terms used in this Certificate have their respective meanings
          set forth in the Pooling and Servicing Agreement.

     2.   First USA is as of the date hereof the Seller and Servicer under the
          Pooling and Servicing Agreement.

     3.   The undersigned are Servicing Officers.

     4.   The aggregate amount of Collections processed for the Due Period for
          this Distribution Date was equal to                                                    $    4,668,909,530.57

     5.   (a)  The aggregate amount of such Collections allocated to Principal
          Receivables for the Due Period for this Distribution Date equal to                     $    4,441,055,351.52

          (b)  The aggregate amount of such Collections allocated to Finance Charge Receivables
          for the Due Period for this Distribution Date was equal to                             $      227,854,179.05

     6.   (a)  The aggregate Interchange Amount (which will be included as Finance Charge
          Receivables for all Series) for this Distribution Date was equal to                    $       71,056,885.62

          (b)  The aggregate Net Recoveries Amount (which will be included as Finance Charge
          Receivables for all Series) for this Distribution Date was equal to                    $        8,861,574.72
</TABLE>

<PAGE>

     7.   The Invested Percentage of Collections allocated to Principal
          Receivables for the Due Period was equal to for:

                    Series 1994-K                      3.221%
                    Series 1994-L                      3.221%
                    Series 1995-M                      3.681%
                    Series 1995-O                      3.681%
                    Series 1995-P                      3.681%
                    Series 1996-Q                      6.626%
                    Series 1996-R                      2.945%
                    Series 1996-S                      5.154%
                    Series 1997-T                      4.417%
                    Series 1997-U                      2.945%
                    Series 1998-V                      7.362%
                    Series 1999-W                      5.522%
                    Series 1999-X                      5.522%
                    Series 1999-Y                      4.049%

                                       2
<PAGE>

     8.   The Invested Percentage of Collections allocated to Finance Charge
          Receivables for the Due Period was equal to for:

                    Series 1994-K                        0.537%
                    Series 1994-L                        0.537%
                    Series 1995-M                        3.681%
                    Series 1995-O                        3.681%
                    Series 1995-P                        3.681%
                    Series 1996-Q                        6.626%
                    Series 1996-R                        1.207%
                    Series 1996-S                        5.154%
                    Series 1997-T                        4.417%
                    Series 1997-U                        2.945%
                    Series 1998-V                        7.362%
                    Series 1999-W                        5.522%
                    Series 1999-X                        5.522%
                    Series 1999-Y                        4.049%

                                       3
<PAGE>

     9.   The Invested Percentage with respect to the Investor Default Amount
          for the Due Period was equal to for:

                    Series 1994-K                      0.537%
                    Series 1994-L                      0.537%
                    Series 1995-M                      3.681%
                    Series 1995-O                      3.681%
                    Series 1995-P                      3.681%
                    Series 1996-Q                      6.626%
                    Series 1996-R                      1.207%
                    Series 1996-S                      5.154%
                    Series 1997-T                      4.417%
                    Series 1997-U                      2.945%
                    Series 1998-V                      7.362%
                    Series 1999-W                      5.522%
                    Series 1999-X                      5.522%
                    Series 1999-Y                      4.049%

                                       4
<PAGE>

     10.  The aggregate amount of drawings or payments, if any, under the
          Enhancement, if any, required to be made on the next succeeding
          Distribution Date is equal to for:

               Series 1994-K                             $0.00
               Series 1994-L                             $0.00
               Series 1995-M                             $0.00
               Series 1995-O                             $0.00
               Series 1995-P                             $0.00
               Series 1996-Q                             $0.00
               Series 1996-R                             $0.00
               Series 1996-S                             $0.00
               Series 1997-T                             $0.00
               Series 1997-U                             $0.00
               Series 1998-V                             $0.00
               Series 1999-W                             $0.00
               Series 1999-X                             $0.00
               Series 1999-Y                             $0.00

                                       5
<PAGE>

     11. The amount of interest due on the Cash Collateral Account loan, if
         applicable, required to be paid on the next Distribution Date is equal
         to for:

          Series 1994-K               $1,685.67
          Series 1994-L               $    0.00
          Series 1995-M               $    0.00
          Series 1995-O               $    0.00
          Series 1995-P               $    0.00
          Series 1996-Q               $    0.00
          Series 1996-R               $    0.00
          Series 1996-S               $    0.00
          Series 1997-T               $    0.00
          Series 1997-U               $    0.00
          Series 1998-V               $    0.00
          Series 1999-W               $    0.00
          Series 1999-X               $    0.00
          Series 1999-Y               $    0.00

                                       6
<PAGE>

     12.  The amount of Monthly Servicing Fee required to be paid on the next
          succeeding Distribution Date is equal to for:

           Series 1994-K          $  104,166.67
           Series 1994-L          $  104,166.67
           Series 1995-M          $  714,285.72
           Series 1995-O          $  714,285.72
           Series 1995-P          $  714,285.72
           Series 1996-Q          $1,285,714.28
           Series 1996-R          $  234,241.68
           Series 1996-S          $1,000,000.00
           Series 1997-T          $  857,142.85
           Series 1997-U          $  571,428.58
           Series 1998-V          $1,428,571.43
           Series 1999-W          $1,071,428.57
           Series 1999-X          $1,071,428.57
           Series 1999-Y          $  785,714.29


                                       7
<PAGE>

     13.  The aggregate amount payable to the Investor Certificateholders on the
          succeeding Distribution Date in respect of interest is equal to for:


               Series 1994-K                  $  521,412.04
               Series 1994-L                  $  496,527.78
               Series 1995-M                  $3,629,365.08
               Series 1995-O                  $3,631,726.19
               Series 1995-P                  $3,601,369.05
               Series 1996-Q                  $6,433,892.86
               Series 1996-R                  $1,162,598.68
               Series 1996-S                  $5,016,845.82
               Series 1997-T                  $4,259,309.53
               Series 1997-U                  $2,859,507.94
               Series 1998-V                  $7,321,255.06
               Series 1999-W                  $5,365,452.51
               Series 1999-X                  $5,447,153.16
               Series 1999-Y                  $3,988,591.27

                                       8
<PAGE>

     14.  The aggregate amount payable to the Investor Certificateholders on the
          succeeding Distribution Date in respect of principal is equal to for:

            Series 1994-K                    $41,666,666.67
            Series 1994-L                    $41,666,666.67
            Series 1995-M                    $         0.00
            Series 1995-O                    $         0.00
            Series 1995-P                    $48,169,557.75
            Series 1996-Q                    $         0.00
            Series 1996-R                    $38,535,642.33
            Series 1996-S                    $         0.00
            Series 1997-T                    $         0.00
            Series 1997-U                    $         0.00
            Series 1998-V                    $         0.00
            Series 1999-W                    $         0.00
            Series 1999-X                    $         0.00
            Series 1999-Y                    $         0.00


                                       9
<PAGE>

<TABLE>
     <S>                                                                                                 <C>
     15.   The excess, if any, of the Seller Amount over the Aggregate Principal
           Receivables required to be maintained pursuant to the Agreement............................   $5,894,757,618.29

     16.   The Seller Amount for the Due Period divided by Aggregate Principal Receivables
           for the Due Period ........................................................................              45.080%

     17.   The Minimum Seller's Interest Percentage...................................................               7.000%

     18.   Attached hereto is a true and correct copy of then statement required to be
           delivered by the Servicer on the date of this Certificate to the Trustee in respect
           of each Series of outstanding  pursuant to Section 5.02(a) of the Agreement, if applicable.


     19.   As of the date hereof, to the best knowledge of the undersigned, no
           default in the performance of the obligation of the Servicer under
           the Pooling and Servicing Agreement has occurred or is continuing
           except as follows:                                                                                         NONE

     20.   As of the date hereof no Liquidation Event has been deemed to have occurred for the Due
           Period for this Distribution Date with respect to any Series.

     21.   As of the date hereof, to the best of the knowledge of the undersigned, no Lien has been
           placed on any of the Receivables other than the Lien granted by the Pooling and
           Servicing Agreement.

     22.   During the preceding calendar month, the number of newly - originated Accounts was.........              15,980
</TABLE>

                                       10
<PAGE>

MONTHLY SERVICER'S CERTIFICATE
Signature Page

IN WITNESS WHEREOF, the undersigned have duly executed and delivered this
certificate the date first set forth above.


                                        First USA Bank, N.A.
                                        As Servicer

                                        By: /s/ Tracie Klein
                                            ---------------------------
                                               TRACIE KLEIN
                                        Title: FIRST VICE PRESIDENT