Lymphoid tissues from 24 human fetuses were assayed for hemolytic plaqueforming cells (PFC) against a variety of erythrocyte targets. PFC against maternal and other erythrocyte antigens were commonly detected in human fetal liver, lymph nodes, spleen, or thymus as early as 16 weeks gestation and were usually more abundant in liver than in spleen after 16 weeks gestation. These data corroborate studies from other laboratories which indicate that human fetuses develop some forms of immunocompetence very early during gestation.