SUMMARYA study of split tolerance was carried out, utilizing C3H male cells as the inducing inoculum, and unrelated females as hosts; tolerance was limited to male-specific (MS) antigens. Split tolerance could be induced as efficiently in 8-day-old hosts as in newborns, whereas 15-day-old hosts were unresponsive to weight-adjusted doses of the same inoculum. The spleens of split tolerant females contained MS antigen, although in exceedingly low concentrations. Abolition of split tolerance could be effected with lymphoid cells whose specificities were directed against the MS antigen, but not by cells sensitized to donor strain allogeneic antigens alone. Induction of split tolerance was prevented in neonatal hosts adoptively immunized to either donor strain allogeneic or MS antigens.