Studies of rabbit major histocompatibility complex proteins have suggested that rabbits express only a single class I antigen, in contrast to most mouse strains (H-2K, D, and L) and man (HLA-A, -B, and -C), which express three. To explore the significance and the molecular basis of this apparent species difference, we have characterized the expressed class I protein and a corresponding cDNA clone from the rabbit cell line RL-5, which is derived from the inbred B/J rabbit strain. As an extension of these analyses, this report documents the genomic sequence of a gene, designated 19-1, which encodes the same histocompatibility antigen expressed in RL-5. The availability of the corresponding full-length cDNA and amino-terminal protein sequence indicates the fully functional nature of the 19-1 gene and allows presumptive assignment of the transcription start site and delineation of exon/intron boundaries. Comparisons of the rabbit gene with homologous human and mouse sequences reveal a striking similarity between 19-1 and human genes in both exon/intron organization and specific nucleotide sequences; this close similarity allows tentative identification of previously unrecognized transcriptional start sites in the human genes.