All mammalian species investigated have a chromosomal region designated as the major histocompatibility complex or m.h.c. The biological significance of the m.h.c. goes far beyond controlling the most important histocompatibility or transplantation antigens; the capacity to respond immunologically, the susceptibility to disease (including cancer), the serum level of several complement factors and numerous other biological traits are regulated by genetic systems closely linked within that chromosomal region. While the basic structure of the m.h.c. seems to be rather similar for all mammalian species, the similarities among the m.h.c. of human and non-human primates are particularly impressive. In this communication, m.h.c. gene products of rhesus monkey, chimpanzee and man are compared and reviewed. Evolutionary aspects of the persistence of the m.h.c. region or 'supergene' throughout the animal kingdom are discussed.