The genomic structure of the joining (J) and constant (C) regions of the locus encoding the beta-chain of the murine T-cell receptor has been analysed. The gene segments are arranged tandemly (J-C/J-C) within a 15-kilobase region. The two constant-region genes are almost identical, differing by only four amino acids, all in carboxy-terminal portions. Each C region comprises four exons encoding an external globular domain, a small hinge-like region, a transmembrane region and a cytoplasmic tail plus 3'-untranslated region. The two clusters of J regions each contain 7 distinct elements, 12 of which may be functional.