The effect of different concentrations of phorbol myristate acetate (PMA) on the development of cytotoxic cells was studied. PMA was selectively able to prevent the development of cytotoxic cells in a mixed leucocyte culture, while allowing the responding cells to proliferate. The higher concentration of PMA (10(-5)M) blocked both direct cytotoxicity and lytic activity in the presence of lectin, while the lower concentration (10(-8) M) only prevented direct lytic function. The removal of PMA and subsequent addition of recombinant interleukin 2 (IL-2) or IL-2-containing supernatants effectively reversed the effect of PMA with recovery of antigen-specific lytic function of cells treated with 10(-8) M, while cells treated with 10(-5)M PMA only recovered lectin-dependent cytotoxic ability.