Progesterone derived either from the corpus luteum or the placenta, or both, has been shown to be essential for the maintenance of pregnancy in every species examined. The essential role of the corpus luteum was discovered by Frankel in 1903, and extracts of the corpus luteum were shown to be effective in maintaining pregnancy in rabbits by Allen and Corner about 30 years later. Many more, recent studies that utilized elegant immunologic and chemical techniques have further elaborated on the importance of progesterone for pregnancy maintenance. However, the functional role of progesterone remains unclear. The pioneering studies of Csapo led to the concept that progesterone dominance of the uterus suppresses uterine contractility until near the end of gestation. However, other effects of progesterone may be equally as important (for an extensive review of progesterone and pregnancy maintenance, see Reference I). More than 20 years ago, it was proposed2 that a'* barrier" exists between fetuses and both normal and sensitized mothers that prevents the mother from developing a state of transplantation immunity to the paternal histocompatibility antigens of the fetus. Much attention has been directed to the possibility that the trophoblast serves this function, simply because it represents the fetal cellular element that, to varying degrees in different species, has direct contact with maternal blood and tissue (decidua). The effectiveness of the placental barrier has been ascribed to nonantigenicity of trophoblastic cells,'secretion of fibrinoid or mucopolysaccharide by trophoblasts that masks histocompatibility antigens, 4s5 and suppression of cellular and humoral immunity by trophoblastic hormones. 6 Considering that pregnancy can be terminated by administration of heterolcgous antiplacental serum, on the one hand, and that interspecies matings have been successful, on the other, it seems obvious that a universal, highly efficient mechanism must prevent the immunologic destruction of the fetus. The idea that trophoblastic hormones may be important in mediating immunologic tolerance of pregnancy has gained considerable interest recently. Depression of maternal cellular immunity, particularly during the third trimester of human pregnancy, has been observed both in vivo7. 8 and in vitro. Thymus-dependent lymphocytic proliferation measured after stim~ lation~~~~ with mitogens (phytohemagglutinin, PHA) or in mixed lymphocyte cultures (MLC)"* I2 is sup-