The ability of small numbers of LN cells to produce cytotoxic lymphocytes on in vitro culture with allogeneic stimulator cells is greatly augmented by the addition of spleen cells from athymic nude mice. The possibility that the synergism is a result of improved culture conditions or a "feeder effect" is excluded. All cytotoxic cells found in these cultures are shown to be T cells and to arise from precursors contained in the LN-cell component. The nude spleen cell component appears to be providing a required non-T cell which has been lost from the LN component through dilution. Synergism between the two components can occur whether they are syngeneic or allogeneic provided that both can recognize the same alloantigens in the stimulator population.