The immunologic paradox of mammalian pregnancy involves the mutual abrogration of rejection by two genetically disparate individuals. The mechanism whereby the fetus survives, despite its marked similarity to a graft in foreign tissue is unknown. This study is designed to elucidate the immunological competence of maternal and fetal lymphocytes during gestation. In addition the role of the placenta as a priveleged tissue which fails to express histocompatibility antigens and thus escapes rejection is also under study. The principles involved this success of nature in preventing rejection of the intrauternine fetus have far- reaching implications for immunological theory in transplantation and tumor immunobiology.