The clonotypic T-cell antigen receptor (TCR)-beta chain contains two extracellular intrachain disulfide bonds. It belongs to the immunoglobulin gene superfamily and is subdivided into variable (V), joining (J), diversity (D) and constant (C) region. Monoclonal antibody (MoAb) KJ25 is believed to recognize an epitope in the V-domain of TCR-beta (V beta 3) chain, but its epitope requirements are unknown. In this study of TCR-alpha beta chain interactions using chimeric recombinant TCR-beta chains, the authors found that partial substitution of the C beta-domain with that of interleukin-2 receptor alpha chain (Tac) sequences led to the loss of TCR-V beta 3 epitope recognition by KJ25. These results suggest that epitope recognition of the TCR-V beta 3 by KJ25 MoAb is dependent not only on the V-domain, but also on the close contact with the extracellular C-domain which influences the conformation and epitope recognition of the V beta 3-region. This may not be unique to V beta 3 and may be a general feature of TCR-beta protein folding.