Five new serologically defined (SD) tissue antigens of rhesus monkeys are described. Results of a population study and a segregation analysis in families were consistent with their control by the major histocompatibility complex (MHC), as alleles of the two previously established SD loci of RhL--A. The number of identifiable SD specificities of the rhesus monkeys' MHC is now twenty-five, thirteen controlled by the SD1 locus and twelve by SD2. The recombination frequency between SD1 and SD2 is estimated to be 0.3%. No evidence of a third SD series, as analogue of the human HLA--C locus, has yet been found.