It has become increasingly evident that the major histocompatibility complexes (MHCs) which have been found in all vertebrates studied so far, have other functions than creating difficulties for transplant surgeons. From studies in experimental animals and in man, it has been discovered1 that the MHC contains (1) genes controlling antigens on cell surfaces, (2) genes controlling some components of the complement cascade, and (3) genes controlling the immune response against certain antigens, the Ir genes.