ABSTRACT: The H-Y (male) antigen is phylogenetically conserved among vertebrate species, including the species man. Previous studies have indicated the presence of a “soluble” H-Y antigen in male serum and culture fluids of male cells. We examined over 50 samples of amniotic fluid from male and female fetuses to determine if H-Y typing could be correlated with the sex of the fetus. Samples of amniotic fluid were tested to inhibit the reactivity of monoclonal antibodies in a standard H-Y assay with protein A sheep red blood cells. We found that amniotic fluids from male fetuses inhibited 40% of the reactivity and that amniotic fluids from female fetuses inhibited 0.5% of reactivity. We could also correctly identify the sex of 90% of male fetuses and 100% of female fetuses. We have not yet identified the exact nature of the inhibiting antigen(s) in the amniotic fluids, but our results clearly indicate the feasibility of fetal H-Y typing.