Allelic diversity at the HLA-OPBl locus was determined by PCR-oligotyping in a sample of 125 healthy Swiss individuals. A total of 17 alleles were detected among which four main alleles (DPBP1*0401, *0201, *0301, *0402) reached a cumulative frequency 0f 74.8%. HLA-A and -B (by serology) and HLA-DRBl (by oligotyping) allelic pOlymorphisms were analysed also. HLA-B and HLA-DRBl loci were highly polymorphic with 25 and 28 alleles respectively and similar heterozygosity levels of 0.93 and 0.92. These two loci were found to be more polymorphic than expected under neutrality, while lower heterozygosity levels were found for HLA-A (0.87) and DPB 1 (0.81) loci. This paper presents also a global comparison of DPBl allelic frequencies among 15 populations from four continents. As opposed to the DRBl locus, overall DPB1 is shown to have a lower level of polymorphism and may be considered as neutral in all tested populations. DPB1 genetic diversity is correlated significantly with geography also, as found previously for DRBl. Two- and four-locus haplotype frequencies were determined and the significance of their linkage disequilibrium tested by an original non-parametric method. A significant positive linkage disequilibrium was found for 11 A-B, 16 B-DRB1, 7 DRBI-DPBl and 3 A-B-DRBI-DPBl haplotypes. The overall linkage disequilibrium between DRB1 and DPB1 was much lower than expected from the physical distance and lower than for A-B and B-DRB 1 pairs. The implications of these results for bone marrow transplantation and for the evolution of HLA loci are discussed.