Several genes were found closely associated with major histocompatibility class I and class II beta-chain genes in chicken genomic DNA clusters by hybridizing tissue-specific cDNA probes to cosmid clones. A cDNA probe for one of these genes, probe C12.3 isolated from a chicken liver cDNA library, was used to clone the homologous sequence H12.3 from a human B-lymphoblastoid cell line cDNA library. C12.3 and H12.3 encode exactly the same 317-residue-long protein. The sequence of 12.3 shows significant homology with the two known guanine nucleotide-binding protein beta subunits (GP beta 1 and GP beta 2) and other proteins that all share the same segmented structure with seven internal homologous repeats about 45 residues in length. Unlike the chicken gene, the human H12.3 gene and its mouse counterpart are not located on the same chromosome as the major histocompatibility complex. A possible involvement of the C12.3 gene product in major histocompatibility complex-linked control of lymphocyte proliferation in chickens is discussed.