SUMMARY Histocompatibility antigens have been detected on melanoblasts and hair follicle cells by means of test grafting of skin from allophenic mice containing two homozygous subpopulations of cells with different alleles at the H-2 locus. Melanoblasts bore a visible color marker (e.g., B/B ↩ b/b) and hair follicle cells were marked at the agouti locus (A/A ↩ a/a). In skin grafts to parental isogenic strains, there was selective survival of those melanoblasts and hair follicle cells whose visible phenotypes were from the same H-2 strain as was the host, while cells of the foreign H-2 strain were destroyed. Genotype-specific homograft rejection by the hosts was highly localized to the homologous cells, even when the genotypic cell strains were admixed within a single hair bulb. Donor grafts had some individual hairs with both black and brown pigment granules attributable to B/B and b/b cells, respectively, or intermediate degrees of agouti banding attributable to presence of both A/A and a/a cells; only one pure strain contributed to such follicles after rejection of incompatible cells. The morphogenesis of mosaic hairs was strikingly modified when one of the two clonal components was eliminated. Hair follicle H-2 antigens in the in situ skin also sometimes differed from H-2 antigens of the contained melanoblasts, e.g., when C57BL/6 pigment cells entered a BALB/c hair and there produced pheomelanin. “Allogeneic inhibition” did not occur in any of these animals despite the presence of diverse H-2 antigenic cell types.