The region responsible for T-cell receptor (TCR)a and chain assembly has previously been shown to reside in their extracellular domains. In an attempt to delineate further the structural requirements for TCRa and p chain assembly, chimeric TCR^ chains with increasing length of constant (C) region and mutant TCR/? chains with C-domain point mutations were constructed. Their ability to assemble with wild-type TCRa chain was evaluated in non-T (COS cells) or T cells. The results reveal that the C^ domain is the binding region to TCRa chain, whereas the intact variable (V), diversity (D) and joining (J) regions with a short C-domain of chain are not sufficient for the TCRa and p chain assembly. The unique interchain disulphide bond between TCRa and P chains is not required for the TCRa^ heterodimer formation.