Mhc-DRB and -DQA1 second-exon and -DRB 3'-untranslated-region nucleotide sequences of three lowland gorillas with no known family relationship with each other and of two HLA homozygous typing cell lines were determined and compared with published primate Mhc-DRB and -DQA1 sequences. Eleven distinct MhcGogo-DRB second-exon sequences were found, which represent the gorilla counterparts of the HLA-DRB1*03, -DRB1*10, -DRB3, -DRB5, and -DRB6 allelic lineages. One Gogo-DRB second-exon sequence does not have an obvious human counterpart and is tentatively designated Gogo-DRBY*01. The gorilla equivalents of the HLA-DRB2 and -DRB8 loci were identified as judged on Mhc-DRB 3'-untranslated-region sequences. In addition, four different Gogo-DQA1 alleles belonging to three different allelic lineages were detected. The Mhc-DRB-DQA1 haplotypes of these gorillas were deduced based on the obtained Mhc-DRB and -DQA1 sequences and the two published Mhc-DRB haplotypes of the lowland gorilla Sylvia. All deduced Gogo-DRB-DQA1 haplotypes show gene constellations different from known HLA-DRB-DQA1 haplotypes, while some of the Gogo-DRB haplotypes presented here contain more DRB genes than the HLA-DRB haplotypes. Based on phylogenetic trees, bootstrap analyses, and the gorilla, chimpanzee, and human Mhc-DRB haplotypes described, we propose that at least two Mhc-DRB loci, here tentatively designated Mhc-DRBI and -DRBII, existed on an ancient primate Mhc-DRB haplotype. The Mhc-DRB1*01, -DRB1*02 (-DRB1*15 and -DRB1*16), -DRB1*03 (-DRB1*03, -DRB1*08, -DRB1*11, -DRB1*12, -DRB1*13, and DRB1*14), and -DRB1*10 allelic lineages and -DRB3 and -DRBY loci probably evolved from the hypothetical primate Mhc-DRBI locus, whereas the present primate Mhc-DRB2, -DRB4, and -DRB6 loci originate from the ancient Mhc-DRBII locus of this core primate Mhc-DRB haplotype.