The structure and chemical nature of the serologically defined (SD) antigens coded by the major histocompatibility complex (RhLA) of rhesus monkeys was studied. The use of specific anti-SD sera allowed the selective isolation of the corresponding antigens from crude antigen preparations. These were obtained by detergent solubilization after incorporation of 3H, 35S, and 14C amino acids in lymphocytes or mitogen-stimulated lymphoblasts. The results indicate that the SD antigens are of proteinaceous nature and are composed of two polypeptide chains with molecular weights of 44,000 and 12,000, the latter being beta 2-microglobulin. No differences in molecular size and subunit composition were detected between antigens of both segregant series. The results are discussed in relation to similar data available for the analogous human and murine histocompatibility antigens.