Amino acid substitutions were introduced into the 45 pocket of HLA-A2.1 to determine the potential role of this structurally defined feature of class I molecules in viral peptide and alloantigen presentation. The 45 pocket lies below the alpha 1-domain alpha-helix and is composed of five amino acids, three of which differ between HLA-A2.1 and HLA-B37. These two class I molecules have previously been shown to have largely non-overlapping peptide-binding specificities. Site-directed mutagenesis was used to replace the hydrophobic residues at positions 24, 45, and 67 in the 45 pocket of HLA-A2.1 with the hydrophilic amino acids found in these positions in HLA-B37. Thus, three single amino acid mutants were produced: 24A----S, 45 M----T, and 67V----S. These mutants were transfected into HMy2.C1R cells and assessed for their ability to present influenza virus matrix M1 57-68 peptide and HTLV-I Tax-1 2-25 peptide to HLA-A2.1-restricted, peptide-specific CTL and to present alloantigens to HLA-A2-allospecific CTL lines. Each of these substitutions in the 45 pocket produced a molecule that failed to present the M1 peptide to most M1 peptide-specific CTL lines. In contrast, none of these mutations affected presentation of the Tax-1 peptide to Tax-1-specific CTL lines, which indicates that these mutant HLA-A2 molecules can function in viral peptide presentation. Two of the three substitutions in the 45 pocket resulted in lack of recognition by a subset of HLA-A2 allospecific CTL lines. These results demonstrate that the amino acid side chains in the 45 pocket can strongly influence peptide presentation and suggest that the 45 pocket may play a role in determining peptide-binding specificity.