SUMMARYIn the rat, a minimum of four genetic loci,Ag-A, Ag-B, Ag-C, andAg-B, determine cellular antigens with different properties of intraspecific immunogenicity, tissue distributions, and histocompatibility associations. Eighteen inbred strains have been classified for the presence of specific alleles. A number of strains were found to exhibit the same antigen at the importantAg-Bhistocompatibility locus. The historical facts available for each strain suggest that the most probable explanations for this situation are: (1) multiple fixation of the same allele by inbreeding from heterogeneous stocks having common ancestry and (2) long-term stability at theAg-Blocus within inbred strains.