A major histocompatibility complex (MHC) has been demonstrated in all mammalian and avian species studied and in several fish and amphibian species. Interestingly, the MHC loci occur in a homologous linkage group in various species. Whether such conservation of a linkage group during the evolution of species is due to chance or is of functional (selective) significance is unknown. Recent analyses of cDNA and genomic clones show the similar overall structure and domain organization of the genes coding for class I and class II antigens of the MHC in various species. Biochemical characterization of the histocompatibility antigens has shown their basic molecular structure to be the same in every species so far examined.