The present study demonstrates that in vitro stimulation by Con A of peripheral blood lymphocytes from certain normal individuals may result in preferential activation of either cytotoxic effector cells capable of killing allogeneic target cells or activation of suppressor cells that prevent the generation of cells cytotoxic to allogeneic targets. Con A-activated lymphoid cells from individuals displaying high levels of cytotoxicity against allogeneic targets (high responders) did not suppress the generation of cytotoxic cells in cell suspensions from other high responders; however, Con A-activated cells from individuals displaying low levels of killing against allogeneic targets (low responders) regularly suppressed the generation of cytotoxic cells in cell suspensions from high responders. Lymphoid cells from low responders suppressed the generation of cytotoxic cells when cocultured in the presence of Con A with cells from a high responder but did not affect the function of killer cells once they had been generated. Pretreatment of cells from a low responder with mitomycin C before co-culturing them in the presence of Con A with lymphoid cells from a high responder impaired their ability to suppress the generation of cytotoxic cells, suggesting that proliferation was necessary for optimal generation of these suppressor cells. In addition, it was demonstrated that Con A-activated cells from low responders were unable to kill an allogeneic target, even when these cells were generated in the presence of a high responder, suggesting that a lack of helper cells was not a sufficient explanation for the inability of cells from low responders to generate killer cells. These studies provide further evidence that suppressor cell functions of subpopulation of lymphoid cells play an important role in the regulation of immune responses of normal individuals.