SUMMARY Both H-X and H-Y loci are demonstrable in inbred strains of rats by slow skin graft rejection in intrastrain male ➝ female (Lewis, Fischer, and Buffalo) combinations and in parental male ➝ reciprocal Fx hybrid male (Lewis and Fischer) combinations. Although these sex-associated antigens regularly functioned as weak immunogens, both H-X alleles yielded more potent immunogens than H-Y alleles, as shown by onset of rejection and survival times of test skin grafts in parental male ➝ Fi hybrid male combinations. Females were more vigorous responders than males to the same H-Y antigens. Differences in responsiveness of Lewis and Fischer females to H-Y antigens are not attributable to their common H-l (Ag-B) allele. Neither H-X nor H-Y specificities were detectable in kidney tissue, since orthotopic grafts survived indefinitely (>300 days), with undiminished function even after pre immunization. Possible reasons for the lack of expression or effect of H-X and H-Y antigens on kidney graft survival are discussed.