The presence of male fetal lymphocytes in the maternal blood of 18 pregnant women (15 primigravidae and three who had had previous pregnancies) was analysed by counting quinacrine positive Y bodies in interphases of Y chromosomes in metaphases. Counts were also performed on a control population of ten young women who had never been pregnant and on another control population of seven men to test the efficiency of the staining technique used. After completion of the calculations, comparison of the results with the sex of the newborn babies showed a correct sex prediction of 83% by interphase scoring and of 89% by metaphase scoring. The lymphocyte transfer from fetus to mother has already started by the tenth week of pregnancy, and the percentage of fetal cell present in the bloodstream of a pregnant woman increases with the duration of the pregnancy.