SUMMARYRat alloantisera from reciprocal immunizations with eight combinations of inbred strains have been subjected to absorption analysis for the presence of Ag-B and non- Ag-B antibodies. Reagents detecting 12 Ag-B specificities were denned and used to characterize six alleles (Ag-Bl-Ag-B6) for which the prototype strains were designated as Lewis, Wistar/Furth, BN, DA, AUG-28807, and Buffalo, respectively. In addition to various cross reactive patterns, the Ag-B antigens determined by each of the six alleles were found to possess distinctive specificities, and these factors were incorporated into theAg-Ballelic terminology. Linkage tests established the independence of the Ag-B genes from those at theAg-Clocus and at four loci which determine coat characteristics. The serological interrelationships of the various Ag-B antigens are discussed in terms of both strain ancestry and possible relevance to interstrain immunogenicity.