Phorbol myristate acetate (PMA) has little immediate effect on the lysis of antigenic tumor targets by the representative cytotoxic T lymphocyte (CTL) clones B6D/2-7c, 5MB6-5, and 5MB10-31. However, prolonged contact (24-48 hr) with PMA (10(-6)M) can profoundly depress the lytic activity of these and other cloned T lymphocytes. This concentration of PMA is neither toxic nor mitogenic for cloned T lymphocytes. B6D/2-7c cells that are treated with PMA lose some ability to bind tumor targets; however, the primary defect in PMA-treated B6D/2-7c cells appears to be at the level of lethal hit delivery, because cells remain essentially ineffective at tumor cell lysis in the presence of agglutinating lectin. Nonetheless, PMA-treated 5MB10-31 and B6D/2-7c cells continue to respond to the proliferative stimuli associated with alloantigens, especially in the presence of exogenous lymphokines. B6D/2-7c cells treated with PMA neither acquire the ability to suppress the cytolytic activity of untreated B6D/2-7c cells, nor undergo any significant alteration of Lyt-2 expression. PMA-induced loss of lytic activity is reversible, and cytolysis is reexpressed by PMA-treated B6D/2-7c cells if they are incubated with 2 degrees MLC SN, but not WEHI-3 SN. The reexpression of cytolysis occurs in the presence of cytostatic concentrations of cytosine arabinoside (ARA-C), indicating that cell proliferation is not required for this process. These data show that cloned CTL are capable of reversible cytotoxic function, and they establish the utility of PMA to probe requirements for expression of CTL function.