The primary purpose of this research was to produce and analyze recombinants of the major histocompatibility complex of the chicken in order to provide useful information concerning its structure and function. Recombinant research provides data relating to the linear order of genes and aids in elucidating gene functions. On a practical level, such knowledge bears on the health and well-being of a species. From the point of view of evolutionary origin, this information complements that already known of the mammalian major histocompatibility complex;From matings of B complex heterozygotes (B('1)B('19)) to homozygotes (B('2)B('2)), 5 recombinants were produced in 5,584 chicks tested by hemagglutination. Analysis of the data indicates that the B('1) parental haplotype contributed the B-G gene and the B('19) parental haplotype contributed the B-F gene to the recombinant haplotypes. The new recombinants have been designated B('19r1), B('19r2), B('19r3), B('19r4), and B('19r5). The B-F region of the B('19r1) haplotype controls allograft rejection, mixed lymphocyte reaction, and graft-versus-host splenomegaly. No functional properties could be assigned to the B-G region;Recombination between a locus controlling immune response to the amino acid polymer GAT and a chromosome segment containing the B-G and B-F genes has previously been reported from Iowa State University studies. Because the B('19r1) haplotype confers high immune response to the amino acid polymer GAT, it was likely produced from a crossover between the B-G locus and a chromosome segment containing the B-F and Ir-GAT-Hi genes. The region controlling immune response to GAT very probably is identical to the previously described B-L region. The putative order of these regions is, therefore, B-G, B-F and B-L.