Rhesus macaque MHC class II DR molecules were isolated from radiolabeled B-cell line extracts by immunoprecipitation with the mAbs 7.3.19.1 and B8.11.2 and subsequently analyzed by 2D-gel electrophoresis. The B-cell lines used for this study were obtained from monkeys that are homozygous for the Mamu-DR region as defined by serologic techniques. Some of these animals have been selectively bred and originate from consanguineous matings. These analyses show that monkeys with the same allotyping may express different types of DR molecules. As in humans, the number of DR molecules expressed per haplotype is not constant and varies from 1 to 3, depending on the serologically defined Mamu-DR specificity, whereas it has been shown that the number of Mamu-DRB genes present per haplotype varies from 2 to 6. Therefore the present study also demonstrates that some of the rhesus macaque DR regions contain one or more pseudogenes.