The major histocompatibility complex (MHC) of domesticated fowl, the B system, is known to contain three subregions which are identified as B-F,B-G and B-L. This invention includes a cDNA clone encoding a B-G antigen of the B system. MHC haplotyping is accomplished by use of novel probes provided by clones to detect restriction fragment length polymorphism (RFLP) patterns typical for various B-G subregion alleles. Additional information concerning this invention is set forth in the attached manuscript entitled Hypervariable sequence diversity in Ig V-like and leucine heptad domains in chicken histocompatibility B-G antigens.