Fetal lymphocytes were sought in the circulation of 16 pregnant women at different stages of gestation. Among 9 pregnancies terminating in the delivery of a male infant, only 4 yielded male cells, thus illustrating the inconsistency of this method (as compared to amniocentesis) for prenatal diagnosis of chromosomal status. In the group of 7 pregnancies, that terminated in the delivery of female infants, 5 showed male cells. Because the 5 women were primiparous, a previous abortion appears the only plausible origin for these male cells. This finding suggests that fetal lymphoid clones can develop in the mother.