The cattle major histocompatibility complex (MHC) region contains a variable number of classical class I genes encoding polymorphic molecules involved in antigen presentation. Six classical class I genes have been described, but assigning sequences to these genes has proved problematic. We propose a refinement of the existing nomenclature, which currently names the 97 known classical class I sequences in a single series. Phylogenetic analysis of the 3' portion of the coding region allows segregation of these into six groups; thus, we have prefixed existing names with the appropriate number. Although it is clear that some of these groups correspond to discrete genes, it is currently not possible to state definitively that all do. However, the main groupings are consistent, and in conjunction with other evidence, we feel it is now appropriate to rename the sequences accordingly. Segregation of sequences into groups in this way will facilitate ongoing research and future use of the cattle MHC section of the Immuno Polymorphism Database.