To better understand the modulatory effects of interleukin-2 (IL-2) on lymphocyte proliferation, we examined the clonality of the in vitro T-cell response by Southern blot hybridization. Tumor-infiltrating lymphocytes (TILs) grown in the presence of IL-2 for 15-26 days had detectable T-cell receptor beta-chain gene rearrangements, which indicated oligoclonal enhancement in culture in four of nine TIL samples. In contrast, none of 11 uncultured TIL samples had detectable gene rearrangements. Lack of detection in at least three of the five negative, cultured TIL samples could be explained by increased numbers of natural killer cells. We hypothesize that the oligoclonal expansion noted results from the enhanced response of immune-primed T cells to IL-2.