Twelve autologous mixed peripheral blood (PBL) tumor cell interactions (MLTC) followed by in vitro expansion of the stimulated T cells in recombinant interleukin-2 (IL-2) were analyzed for the potential emergence of oligoclonal or monoclonal T cell populations in the PBL by Southern blot analysis of the T cell receptor (TCR) beta gene. The emergence of oligoclonal or monoclonal TCR beta gene rearranged populations was seen in 5 of the 12 cases. In 2 of these 5 cases only one dominantly rearranged band was observed. The emergence of oligoclonal or monoclonal T cell populations following stimulation with autologous melanoma cells was associated with predominant CD4 phenotype of the stimulated PBL exhibiting a varied degree of cytotoxicity toward the respective autologous melanoma cells. The evidence of emergence of monoclonal or oligoclonal T cell populations following stimulation with autologous tumor cells strongly supports the existence of T cell-mediated responses against autologous melanomas. Furthermore, cellular and molecular analyses of T cell responses in autologous mixed lymphocyte tumor cell interactions will provide valuable information on the nature of the T cell responses and on the pattern of gene segment usages by the T cells in response to the autologous tumor cells.