A multitude of different antigens can be recognized by T cells through specific receptors. Both the α- and β-chains of the T-cell receptor contribute to the antigen recognition portion1. The repertoire of β-chain variable region (Vβ) gene segments is limited to some 20 elements2-4 which seem to be used randomly in different T cells2,5,6. Diversity at the β-chain level can be created in several ways2,3,7-12: a multiplicity of germline gene segments; combinatorial diversity by rearranging different V, diversity (D), joining (J) and constant (C) region elements; junctional diversity by joining gene segments at different sites; N-region diversity, that is, insertion of random nucleotides at junctional sites; and somatic mutation13. However, the major sources and the extent of diversity of the T-cell receptor are unclear. To address this issue, 42 H-2Kb-restricted, 2,4,6-trinitrophenyl (TNP)-specific cytotoxic T-cell (Tc) clones from C57BL/6 mice were characterized with respect to expression of different β-chain gene segments in messenger RNA using specific oligonucleotide probes. We report here that nearly half of the Tc clones use identical elements for productive β-chain gene rearrangement. Thus, there is a restriction in the use of β-chain gene segments in this panel of Tc clones which favours a particular Vβ-Dβ-Jβ-C β combination with a defined Dβ element.