Cotransfection of cDNA encoding the trans-activator gene product of human T-cell leukemia virus, type I (HTLV-I) (tat-I), which acts in trans to augment viral gene expression, has revealed strong regulatory effects of this viral protein on the inducible cellular promoters governing human interleukin 2 (IL-2) and IL-2 receptor (Tac) gene expression. The tat-I protein stimulates a 3- to 6-fold increase in IL-2 receptor (Tac) promoter activity in transfected Jurkat T cells, but not in the natural killer-like YT cell line, as measured by changes in the expression of the chloramphenicol acetyltransferase (CAT; EC 22.214.171.124) reporter gene linked to this promoter. In contrast, tat-I alone has little or no effect on IL-2 promoter activity in Jurkat T cells but markedly synergizes with other mitogenic stimuli (phytohemagglutinin, phorbol 12-myristate 13-acetate, or the OKT3 monoclonal antibody), which alone are ineffective. The tat-I protein also partially circumvents the pronounced inhibitory effects of cyclosporin A on the IL-2 promoter. Other cellular and viral promoters are unaffected by the tat-I gene product, either alone or in combination with other mitogens. The specific effects of the tat-I gene product on the IL-2 and IL-2 receptor (Tac) promoters suggest the possibility of an autocrine or paracrine mechanism of T-cell growth as an early event in HTLV-I-mediated leukemogenesis.