Microcapillary HPLC electrospray ionization tandem mass spectrometry was used to sequence 15 peptides eluted from HLA-B7. Sequence alignment implicated four peptide positions in specific interactions with the class I molecule, and their importance was confirmed using synthetic peptides. Because no crystal structure for HLA-B7 was available, computer-assisted modeling was used to understand novel aspects of peptide binding specificity and to accurately predict the effect of defined changes in peptide structure. The results demonstrate that mass-spectrometric sequencing coupled with computer-assisted modeling can be used in the absence of a crystal structure to make accurate predictions concerning requirements for peptide binding to class I molecules. These techniques may be valuable to predict or engineer T cell epitopes.