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In a population of (CB X CC)F1 X WB hybrids, a chicken was found with a presumably recombinant haplotype, BR1, whose antigenic products detectable by hemagglutination contained determinants derived from both parental haplotypes, i.e. B1 (from CB) and B2 (from CC). This recombinant bird and its progeny from different crosses were tested by skin grafting, graft-vs.-host (GVH) and mixed lymphocyte reactions (MLR). The results define two regions of the B system, the B-F and the B-G. The B-F region determined serologically defined antigens, histocompatibility antigens, and controlled the GVH and MLR reactions, while the B-G region was responsible only for synthesis of serologically detectable antigens.