Superantigens (SAgs) are viral or bacterial proteins that act as potent T-cell stimulants and have been implicated in a number of human diseases, including toxic shock syndrome, diabetes mellitus and multiple sclerosis. The interaction of SAgs with the T-cell receptor (TCR) and major histocompatibility complex (MHC) proteins results in the stimulation of a disproportionately large fraction of the T-cell population. We report here the crystal structures of the beta-chain of a TCR complexed with the Staphylococcus aureus enterotoxins C2 and C3 (SEC2, SEC3). These enterotoxins, which cause both toxic shock and food poisoning, bind in an identical way to the TCR beta-chain. The complementarity-determining region 2 (CDR2) of the beta-chain and, to lesser extents, CDR1 and hypervariable region 4 (HV4), bind in a cleft between the two domains of the SAgs. Thus, there is considerable overlap between the SAg-binding site and the peptide/MHC-binding sites of the TCR. A model of a TCR-SAg-MHC complex constructed from the crystal structures of (1) the beta-chain-SEC3 complex, (2) a complex between staphylococcal enterotoxin B (SEB) and an MHC molecule, and (3) a TCR V(alpha) domain, reveals that the SAg acts as a wedge between the TCR and MHC to displace the antigenic peptide away from the TCR combining site. In this way, the SAg is able to circumvent the normal mechanism for T-cell activation by specific peptide/MHC complexes.