The immunologic theory of aging proposes that the normal process of aging in man and all animals is pathogenetically related to faulty immunological processes and may be analogous to a type of autoimmune phenomena ultimately involving all body tissues. It may be said that the sharply increased incidence in elderly humans of the autoimmune and immunodeficiency "diseases of age" are thought to be greatly potentiated by the age-related decline in immune surveillance mechanisms particularly involving self/non-self discriminatory abilities. The major histocompatibility complex has emerged as a complex of "supergenes" coding for antigens whose ultimate biological function may be to serve as recognition units allowing lymphocytes to recognize self from non-self on an immunological basis. Also, recent data are consistent with our supposition that differences in age-specific peaks of various immune functional parameters in genetically homozygous mice may be influenced by genes linked to the major histocompatibility complex. These differences may account, at least in part, for the highly strain-dependent, age-specific incidence of certain diseases, including autoimmune and malignant diseases in the mouse. Heightened susceptibility to develop a particular disease in a susceptible animal occurs when a certain balance is reached between the interplay of immune functional parameters which mature, differentiate, or decline at different rates in the same animal. The age-specificity of this balance may be under partial control of H-2 or HLA-linked genes.