A high association has been found between D-locus determinants defined by cellular methods (typing cells) and the serologically defined B cell or Ia antigens controlled by the Ia1 locus. Data from recombinant offspring in two rhesus families place the Ia1 locus within RhLA, in the close vicinity of the D or major MLC locus. In fact, identity between the products of those two loci cannot be excluded. Further, it was shown that cells proven to be homozygous for Ia1 antigens, are nearly always typing cells (and therefore homozygous for D-locus determinants) and, conversely, that proven typing cells carry only a single Ia1 antigen and are thus probably homozygous for Ia1 products. In addition, it could be demonstrated that matching for Ia1 antigens greatly facilitates the detection of unrelated individuvals who are mutually non-reactive in MLC. This "predictive value" of Ia1 matching has important implications for the selection of optimal donors for organ transplantation.

Cytomegalovirus was isolated from chimpanzees. The chimpanzee CMV showed a strong antigenic relationship with human CMV. The genome of the chimpanzee CMV was found to have a molecular weight of 147 +/- 11.3 X 10(6) and showed partial homology to human CMV DNA.

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D‐locus identical pairs of monkeys were alloimmunized. An exchange of skin and blood between initially MLC‐negative unrelated monkeys frequently led to strong mutual reactivity in mixed cultures. This was not the case when RhLA identical siblings were similarly (mutually) immunized. These data led to the postulation of a second RhLA‐linked MLC locus, named D', which exerts its influence only after alloimmunization. Interestingly, strong MLC responsiveness between the RhLA identical siblings could be provoked after further immunization with tissue from a third party animal. This unexpected MLC‐reactivity is ascribed to the influence of minor non MHC linked loci which express themselves if the responder has been sensitized against a disparate D'‐product. It is suggested that the postulated D' locus has a “helper function” in that it assists in triggering proliferative responses in mixed cultures against the products of these non MHC loci.