The DRB gene of the domestic cat MHC appears to be highly polymorphic, with 71 alleles provisionally reported, based on exon 2 sequence. However, these alleles were reported prior to the adoption of strict criteria for allele identification. In this study, we investigated FLA-DRB exon 2 polymorphisms in a cohort of 33 British domestic cats by polymerase chain reaction (PCR) and clonal sequence analysis. Applying the strict criteria for assigning new alleles as used by the established mammalian MHC nomenclature committees, we defined 13 FLA-DRB alleles, including four previously unreported alleles. We identified many sequences that were one or two base pairs different from these 13 defined alleles, and have shown that these are most likely artefacts of PCR amplification. When the same criteria for allele acceptance were applied to the remaining previously reported sequences, 11 further alleles were confirmed. This suggests that to date there is good evidence for 24 FLA-DRB alleles fulfilling nomenclature criteria. Analysis of these 24 alleles reveals a similar pattern of MHC polymorphism to that seen in other mammals, with three regions of hypervariability.