To investigate whether the classical HLA MHC class I loci have been preserved during evolution of the primates, we have cloned, sequenced, and expressed eight MHC class I cDNA from orangutan and gibbon lymphocytes. Both the HLA-A and -B loci are present in both of these species. In fact, lymphocytes from the orangutan expressed three HLA-B-related gene products, suggesting that the ancestral homologue of the HLA-B locus had undergone a duplication in this species. Interestingly, several amino acid motifs thought to be important in the Ag-presenting function of MHC class I molecules were preserved in the Ag-recognition sites of the orangutan and gibbon MHC class I molecules. Finally, these findings suggest that the recombination event between the HLA-A and -E loci occurred over 38 million years ago. These data indicate that the HLA-A and -B loci are extremely stable and that recombination between them is rare. Furthermore, the data presented here argue against the role of concerted evolution in the evolution of primate MHC class I molecules.