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Superantigens (SAgs) are a class of disease‐causing and immunostimulatory proteins of bacterial or viral origin that activate a large number of T‐cells through interaction with the Vβ domain of T‐cell receptors (TCRs). In this study, recombinant TCR β chains were constructed with human variable domains Vβ5.2, Vβ1 and Vβ2.1, expressed as inclusion bodies, refolded and purified. The Streptococcus pyogenes SAg SSA‐1 was cloned and expressed as a soluble periplasmic protein. SSA‐1 was obtained both as a monomer and a dimer that has an intermolecular disulfide bond. We analyzed the biological activity of the recombinant SAgs by proliferation assays. The results suggest that SSA dimerization occludes the TCR interaction site. Naturally occurring SSA dimerization was also observed in supernatants of S. pyogenes isolates. An SSA mutant [SSA(C26S)] was produced to eliminate the Cys responsible for …