The expression and function of several proto-oncogenes were examined in a human acute T cell leukemia line, JURKAT, during phorbol ester-induced terminal differentiation. Treating JURKAT cells with the phorbol ester tetradecanoyl phorbol acetate (TPA) inhibited their proliferation and induced expression of the gene for the interleukin 2 receptor alpha chain (IL2R-alpha), consistent with previous reports. In unstimulated proliferating JURKAT cells, high levels of C-MYC, N-RAS, and BCL2 mRNAs were found that diminished rapidly following TPA-induced cessation of growth. In contrast, accumulation of mRNAs for the C-FOS, C-JUN, and EGR-1 genes increased markedly in TPA-treated cells and preceded the induction of IL2R-alpha mRNA. Expression of C-MYB, C-RAF-1, C-LCK, C-FYN, and C-FGR proto-oncogenes was relatively unchanged. To explore directly the function of two of these protooncogenes in regulating the growth of JURKAT T cells, we stably transferred C-MYC and BCL2 expression plasmids into these cells. Despite sustained expression of C-MYC, BCL2, or the combination of these protooncogenes, TPA continued to inhibit JURKAT cell growth and to induce IL2R expression. Thus, although C-MYC and BCL2 proto-oncogene expression correlated with proliferation in TPA-treated JURKAT cells, continuous over-expression of even the combination of these oncogenes was insufficient for abrogating the effects of TPA in these leukemic T cells. Because human lymphoid malignancies frequently contain chromosomal translocations that deregulate the expression of C-MYC and BCL2, our findings could have relevance for attempts to induce terminal differentiation of leukemic cells by in vitro exposure of patients' bone marrow cells to phorbol esters.