Melanoma differentiation antigens, such as MART-1/MelanA and gp100/PMel17, frequently are observed as targets of tumor infiltrating lymphocytes (TIL) originated from HLA-A*0201-expressing patients with melanoma. Furthermore, particular clinical relevance was attributed to gp100/pMel17 based on the impression that the adoptive transfer of gp100-recognizing TIL was associated with clinical responses in a small group of patients. However, the actual frequency of specific T cells for these melanoma differentiation antigens has never been directly enumerated in ex vivo or in vitro expanded TIL cultures. Here, we enumerated melanoma differentiation antigen-specific T-cell precursor frequency in TIL using tetrameric HLA/epitope complexes, functionally characterizing their responsiveness to cognate epitope by cytokine release assay. T-cell precursor frequencies were enumerated in 11 fresh-tumor preparations and 17 TIL adoptively transferred into patients bearing HLA-A*0201. MART-1 or gp100-specific T cells could be detected respectively in 5 and 2 of the 11 fresh preparations and in 5 and 2 of the 17 adoptively transferred TIL. With one exception, melanoma differentiation antigen-specific T-cell precursor frequency in fresh material and TIL ranged between 5,000 to 21,000/10(6) CD8+ T cells. T-cell precursor frequency was not significantly higher in TIL whose administration was associated with clinical response. These data provide direct enumeration of MART-1/MelanA and gp100/pMel17 reactivity ex vivo and in vitro in the context of HLA-A*0201.