The fluorescence of the human Y chromosome after staining with quinacrine derivatives was used in an attempt to determine the prevalence of XY lymphocytes presumptively emanating from male fetuses in the blood of pregnant women. Seven women who subsequently gave birth to male infants had 0.03%-0.20% lymphocytes with a Y body. In two women pregnant with male fetuses, no Y bodies were detected. Of 12 women who gave birth to girls, eight had no Y bodies. In the remaining four, fluorescent spots were interpreted as Y bodies. One of those women had previously had an abortion. In the remaining three cases, it was found in retrospect that the fluorescent spots emanated from exceptionally brightly fluorescent autosomal regions in the maternal or paternal chromosomes, or both. Phytohemagglutinin-stimulated and unstimulated lymphocytes showed similar frequencies of Y body. Based on determinations of the incidence of Y body-containing lymphocytes in males (35%-66.5%), it was estimated that approximately 0.1%-0.5% of lymphocytes in the blood of most women pregnant with a male fetus are of fetal origin. This figure is remarkably high, since it exceeds in relative terms that of fetal erythrocytes (which mostly appear in the mother’s blood at the time of delivery) by several orders of magnitude. It is suggested that fetal lymphocytes actively cross the feto-maternal barriers. This phenomenon may have a profound influence on the immunologic interactions between fetus and mother. The method described may not turn out to be profitable in the assessment of the fetal sex, since its accuracy may be of the order of only 80%-95%. Moreover, several limitations prevail.