Defects in T cell function are known to be present in a subset of patients with CVID, but the true nature of these defects still has to be revealed. In prior studies we described that T cells from these patients show an impaired proliferative response following activation with recall antigens (E. coli, Tet. Tox., TBE and PPD). Gene expression of IL2 and IFN-gamma in patients' T cells following antigenic stimulation was significantly reduced compared to controls, while IL-2R transcripts were normal. To further characterize the defect we examined T cell responses to bacterial enterotoxins, collectively termed superantigens. Following stimulation with optimal (10 ng/ml p < 0.05) as well as suboptimal (1 ng/ml p < 0.0025) concentrations of staphylococcal enterotoxin A (SEA), proliferative response and cytokine release (IL-2 and IFNg) were significantly decreased in patients' T cells as compared to controls'. When patients' T cells were stimulated with staph. enterotox. C3 (SEC3) an even more pronounced difference between patients' and controls' T cells could be observed (10 ng/ml p < 0.002, 1 ng/ml p < 0.0005). Our data indicate that, in addition to the defect in antigen-induced T cell activation, T cells of CVID patients express a broader impairment in the interaction between the antigen presenting cell and the TCR.