The Melan-A/MART-1 gene product is frequently recognized by tumor-specific HLA-A2-restricted CTL. An immunodominant nonapeptide has been localized to the region spanning residues 27-35. However, the decapeptide including residues 26-35 (the nonapeptide extended NH2 terminally by one residue) appeared to be recognized as efficiently as the nonapeptide. In this study, we show that the optimal length immunodominant peptide appears to correspond to the decapeptide 26-35, as assessed by quantitative analyses of both 4 polyclonal and 13 monoclonal populations of specific CTL. Functional assays of peptide binding to HLA-A2 indicate that the decapeptide is significantly a more efficient binder than the nonapeptide. Moreover, analogues of the decapeptide including substitutions at a secondary HLA-A2 peptide anchor further improve decapeptide binding. Finally, we show that the functional (9 CTL clones analyzed) and structural TCR repertoire (7 CTL clones) of a group of specific CTL clones is rather diverse. The findings reported here may have important implications for future peptide-based melanoma vaccination trials as well as for the monitoring of specific CTL responses in vivo.