Soluble H-Y antigen is taken up by cells of the homogametic gonad of cattle, dog, chicken and South African clawed frog. After in vitro exposure to mouse testis supernatant or male fetal calf serum, XX ovary cells or ZZ testis cells, which are normally H-Y-, acquire the H-Y+ (H-W+) phenotype and absorb mouse H-Y antibody in standard serological assays. In addition, H-Y antigens of the different species can compete for attachment to target cells of a single species. In a new competitive binding radioassay, uptake of tritiated human H-Y is blocked in XX bovine fetal ovarian cells exposed to non-labeled H-Y of mouse or fetal bull. Because of H-Y antigens of the different species are cross-reactive serologically, positive reaction of H-Y from one species with gonadal cells of another signifies structural conservatism of the H-Y/H-W gonadal "receptor". It follows that establishment of the H-Y/H-W-receptor complex is a common and critical early event in primary sex differentiation of the vertebrates, directing the initially indifferent embryonic gonad towards the heterogametic mode, which may be testicular or ovarian, depending on the species.