In general, the human acute T lymphocytic leukemias are composed of malignant expansions of immature T cells lacking membrane receptors for T cell growth factor (TCGF, interleukin 2) and significant immunoregulatory activity. We investigated whether cultured acute lymphocytic leukemic T cell lines can be induced to differentiate and express the Tac antigen, a cell surface protein that contains a TCGF-binding site, after exposure to phorbol 12-myristate 13-acetate (PMA) and/or phytohemagglutinin (PHA). Reactivity of anti-Tac with induced leukemic T cells was studied by three techniques, including: 1) flow microfluorometry; 2) specific binding of [3H]anti-Tac; and 3) receptor immunoprecipitation with anti-Tac and analysis by SDS-PAGE. After exposure to PMA with or without PHA, both JURKAT and HSB-2 acute lymphocytic leukemic T cells displayed Tac antigen within 6 to 8 hr. Induction of receptor expression was blocked by actinomycin D, suggesting a requirement for new mRNA transcription. Induced JURKAT cells contained approximately 7000 Tac molecules per cell, and the binding of anti-Tac to these cells was blocked in a dose-related manner by purified TCGF but not by insulin or purified recombinant interferon-alpha. SDS-PAGE analysis of anti-Tac immunoprecipitates demonstrated that receptors present on induced JURKAT cells were 2000 to 3000 daltons smaller than those present on PHA-activated normal lymphoblasts or induced HSB-2 cells. Induction of JURKAT cells with both PHA and PMA resulted in marked secretion of TCGF as well as the appearance of Tac antigen. After activation of these cells with PMA alone, Tac antigen was similarly expressed, but the level of TCGF synthesis was less than 1% of that obtained after dual induction with PHA and PMA. These data indicate that the signals required for TCGF synthesis and Tac expression are not identical, and furthermore that induction of Tac antigen and TCGF is not obligately linked in these cells.