Malignant melanomas are often infiltrated by T lymphocytes. It is postulated that the presence of tumor-infiltrating lymphocytes (TIL) reflects ongoing immune responses against transformed cells. Such "responses" appear generally inefficient with the potential exception of infrequent clinical situations characterized by spontaneous tumor regression. We have characterized here the molecular structure of the T cell receptor beta chain expressed by TILs in a case of regressive melanoma. Advantage was taken of the PCR technology to study T lymphocytes directly without cell culture. Experimentally validated V beta subfamily specific primers were used to evaluate the V beta usage in TILs and control samples. Our results reveal that clonal T cell populations, precisely defined by their V-D-J junctional sequences, are amplified at the tumor site. The existence of such local antigen-driven selections support the hypothesis that antitumor responses may indeed take place in regressive melanoma.