The effects of recombinant interleukin-2 on resting T lymphocytes were examined in vitro. T cell enriched populations were isolated from the peripheral venous blood of three normal individuals by Ficoll-Hypaque gradient separation and standard sheep erythrocyte rosetting techniques. Interleukin-2 at concentrations of 100 and 1,000 U/ml stimulated tritiated thymidine incorporation, Tac antigen expression, interferon production, and blast transformation in T cell cultures. These changes were observed in the absence of cell proliferation. The effects of interleukin-2 at a concentration of 100 U/ml were substantially blocked by the addition of a 1:1,000 dilution of anti-Tac antibody to the culture medium indicating that the effects observed were mediated by the Tac receptor. These results indicate that recombinant interleukin-2 can have demonstrable functional and morphologic effects on unstimulated human T cells.