SUMMARYA panel of inbred mouse strains selected for their low incidence of common ancestrywas typed for the alleles of theH-1, H-3, H-4, H-7, H-8, H-9, H-12andH-13histocompatibility loci, using histogenetic techniques. As in a previous study, typing revealed new alleles at all loci with the exception of H-l. Because of the inability to type for the null antigen of the H-1ballele, we could not distinguish between strains possessing H-1band those possessing new alleles. In view of the rather high frequency of new alleles (66%), it seems likely that a high degree of genetic polymorphism exists at these histocompatibility loci. The strengths of the non-H-2 antigens varied from the relatively strong H-4b antigen to the extremely weak H-4a antigen. The immune response of the 13 F1groups was rather tightly grouped. No one appeared particularly strong or weak as compared to the group as a whole.