A panel of congenic resistant strains, B10.BY, B10.129(5M), B10.C(41N), B10.C3H(40NX), and B10.D2(58N), were found to differ from their background strain, C57BL/10, at theH-1locus. These strains were typed for thea, b, andc, alleles of theH-1locus using C3H (H-1a), C3H.K (H-1b), and C57BL/10 (H-1c) as test strains. Strains B10.C3H(40NX) and B10.D2(58N) in addition to C3H were found to possess theH-1aallele. Strains B10.129(5M), B10.BY, and B10.C(41N) in addition to C3H.K were found to possess theH-1ballele. As expected no strain tested possessed theH-1callele. Congenic resistant strain-pairs C57BL/10-B10.C(41N) and B10.129(5M)-B10.D2(58N) were found to differ by one histocompatibility gene each. Strain B10.129(5M) seemed also to differ from C57BL/10 at only one histocompatibility locus though with some suggestion of the presence of a very weak contaminant. Strain B10.C3H(40NX) was found to differ from B10.129(5M) and consequently C57BL/10 by two histocompatibility genes, theH-1locus and a relatively strong contaminant histocompatibility gene not linked to the gene for albinism. Grafts from strains with theH-1ballele to congenic strains with allelesH-1aorH-1cwere almost never rejected even in immunized hosts. The end product of this allele appears nonantigenic under the conditions of these tests.