The cDNA and a partial genomic sequence of a rat class I major histocompatibility (RT1) gene, ll/3R, is reported here. The sequence contains several unique amino acid residues at certain positions, mutations in exon 7 (which is not expressed), a mutation of the canonical exon 8 stop codon to a sense codon, and includes a long 3' untranslated region (utr). The structure of exert 7 differs from that found in most rat class I genes and resembles exert 7 of most H-2K, D,L,Q genes. Parts of the 3' noncoding region are homologous to the RT1.A-4 and certain H-2 genes. Expression is detectable by northern blot analysis in mitogen-stimulated lymphocytes only, by polymerase chain reaction (PCR) in each tissue tested. After transfection into L cells ll/3R can be shown to be expressible at the cell surface. Probes derived from the 3' noncoding part crosshybridize with a number of restriction fragments which map to the RT1.C region, thus defining a subfamily of RT1. C region genes. Several members of this subfamily are deleted in the Iml RT1 mutant. The ll/3R gene presents typical features of a class Ib gene. Aspects of evolution and the potential function of the gene are discussed.