Transplantation tolerance of the H-Y antigen may be induced and transferred by lymphoid cells, but not by nonlymphoid cells. In this study we have demonstrated that amongst lymphoid cells, T cells—but not B cells or macrophages—are capable of inducing transplantation tolerance. In contrast, all three populations express sufficient amounts of immunogenic H-Y antigen to sensitize adult female mice to a subsequent male skin graft, as demonstrated by the second-set reaction.