Prolonged contact with nanomole to micromole concentrations of the tumor promoter phorbol myristate acetate causes a significant reduction in the lytic activity of cloned cytolytic T cells. Diminished lysis is apparent even in the presence of agglutinating lectins. PMA does not exert this effect by minimizing clone viability. In fact, these concentrations of PMA cause significant potentiation of antigen-driven proliferation for many of these same clones. The PMA-mediated loss of cytolysis is reversible. Although contact with antigen does not induce or enhance reexpression of cytolysis, PMA-treated cytolytic T cell clones display normal cytolytic activity after contact with lymphokines.