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 SD1locus and twelve by SD2. The recombination frequency between SD1and SD2is estimated to be 0.3%. No evidence of a third SD series, as analogue of the human HLA—C locus, has yet been