Superantigens are the most potent T-cell mitogens so far described, and are believed to activate virtually all the T lymphocytes bearing the appropriate V beta segment in their T-cell receptor (TcR). In order to determine whether the expression of the identical V beta gene confers the same pattern of responsiveness to bacterial superantigens, we have established a panel of 20 untransformed human T-cell clones expressing the V beta 6.7a gene in their TcR. The V beta usage was defined by immunostaining, using the V beta 6.7a-specific monoclonal antibody (mAb) OT145, and confirmed by DNA sequencing of the beta-chain. Although all the clones analysed expressed the same V beta gene, they had disparate patterns of proliferation to challenge with a panel of bacterial enterotoxins. This study demonstrates that the mere expression of the same V beta region by T lymphocytes does not grant an indistinguishable responsiveness to bacterial superantigens. Thus other, as yet undefined, T-lymphocyte components play a key role in the process of T-cell activation induced by bacterial superantigens, influencing the effects mediated by exogenous superantigens on human T cells.