Modifications in tumour antigen-derived epitopes that stabilize the major histocompatibility complex (MHC)-peptide complex result in enhanced stimulatory capacity and improved immunogenicity of the altered peptide. These epitope analogues are attractive candidates for the development of peptide-based vaccine trials. Any modification, however, in tumour antigens may induce T-cell responses that could either fail to react against the naturally occurring peptides or represent only a subset of the total antigen-specific repertoire. In the present study, we performed a critical analysis of the ability of cytotoxic T-lymphocyte (CTL) clones, derived from two melanoma patients through stimulation with the A27L peptide analogue, to cross-react with the naturally processed Melan-A/MART-1 (Melan-A) peptides in terms of T-cell receptor (TCR) affinity, functional avidity and fine antigen specificity. We found that all the A27L-specific clones analysed possessed a very low avidity for the natural Melan-A peptides, and that their binding affinity for human leukocyte antigen (HLA) tetramers complexed with both the modified and the natural Melan-A peptides did not strictly correlate with their functional avidity. We also observed that these clones were able to cross-recognize both natural Melan-A peptides in one patient, but only one peptide in the second patient. We discuss the capability of the A27L peptide analogue to stimulate all the available Melan-A-specific repertoire.