Mixed cultures of DNP-primed spleen cells from old and young BC3F1 mice in diffusion chambers yielded significantly fewer secondary PFC than expected from the numbers of PFC found when young and old spleen cells were cultured alone. There was a negative correlation between the log of the number of PFC found in cultures of old spleen cells alone and the degree of immunosuppression exerted by the cells. The immunosuppressive effect was abolished by pretreatment of the old spleen cells with anti-theta serum and complement. Thus, the effect was attributed to the presence of large numbers of suppressor T cells in the spleen of primed old mice. No suppression was found in mixed cultures of spleen cells from old unimmunized mice and young primed mice.