A three-step reverse hybridization assay for allele level-resolution DRB1-DRB3-DRB4-DRB5 genotyping is described. Samples are initially amplified using a generic primer pair for all DRB1-DRB3-DRB4-DRB5 alleles and PCR products are hybridized to generic typing membranes. An intermediate-resolution level genotyping is obtained at this point. Depending on the phenotype, samples are then subjected to a DR1, DR2, DR4, DR52A, DRB3 and/or DRB5 type-specific amplification and hybridization. A third step, involving sequence-specific PCR followed by type-specific hybridization, is only performed to solve certain DR4 and DR52A heterozygous combinations. The assay allows 100% allele level-resolution DRB genotyping. Hybridization membranes contain panels of SSO probes that were optimized to all react specifically under identical stringency conditions. A computer program was written to assist in analysis of the hybridization patterns. The assay was throughly evaluated and has been used to type over 10,000 donors from the Canadian Unrelated Bone Marrow Donor Registry (UBMDR) at allele level-resolution. This method proved to be flexible, easy to update for newly described alleles, easy to perform, fast, and safe. It is also reliable and specific, as 9 novel DRB alleles so far have been detected as aberrant hybridization patterns.