he placenta has several crucial functions. It is a fetal respiratory organ, a sophisticated endocrine unit, and a membrane that allows preferential and selective transfer of substrates from the mother to the fetus for fetal growth and development. The fact that most maternal tumors do not metastasize to the fetus emphasizes an additional role of the placenta in most circumstances — that of an effective barrier between mother and fetus. However, the involvement of the placenta, and sporadically the fetus, by hematologic tumors and melanoma indicates that the placental barrier is imperfect. Indeed, transfer of fetal cells into the maternal circulation is common and probably occurs throughout gestation in all pregnancies. Much of our knowledge of this phenomenon derives from studies of maternal immunization by red-cell antigens and of erythroblastosis fetalis. This work has shown that as little as 0.1 ml of Rh-positive cells from a fetus can immunize an Rh-negative mother.5 Less common is massive hemorrhage from the fetus into the maternal circulation, which causes profound fetal anemia, nonimmune hydrops fetalis, and stillbirth.6 Even very early in pregnancy, fetal cells can be identified in the maternal circulation. Technology allowing the separation of fetal cells from maternal blood and the analysis of the genes in these cells is soon likely to revolutionize the approach to prenatal diagnosis.