Streptococcal toxic shock syndrome (STSS) is a highly lethal, acute-onset illness that is a subset of invasive streptococcal disease. The majority of clinical STSS cases have been associated with the pyrogenic toxin superantigens (PTSAgs) streptococcal pyrogenic exotoxin A or C (SPE A or C), although cases have been reported that are not associated with either of these exotoxins. Recent genome sequencing projects have revealed a number of open reading frames that potentially encode proteins with similarity to SPEs A and C and to other PTSAgs. Here, we describe the cloning, expression, purification, and functional characterization of a novel exotoxin termed streptococcal pyrogenic exotoxin J (SPE J). Purified recombinant SPE J (rSPE J) expressed from Escherichia coil stimulated the expansion of both rabbit splenocytes and human peripheral blood lymphocytes, preferentially expanded human T cells displaying Vβ2, -3, -12, -14, and -17 on their T-cell receptors, and was active at concentrations as low as 5 × 10-6 μg/ml. Furthermore, rSPE J induced fevers in rabbits and was lethal in two models of STSS. Biochemically, SPE J had a predicted molecular weight of 24,444 and an isoelectric point of 7.7 and lacked the ability to form the cystine loop structure characteristic of many PTSAgs. SPE J shared 19.6, 47.1, 38.8, 18.1, 19.6, and 24.4% identity with SPEs A, C, G, and H, streptococcal superantigen, and streptococcal mitogenic exotoxin Z-2, respectively, and was immunologically cross-reactive with SPE C. The characterization of a seventh functional streptococcal PTSAg raises important questions relating to the evolution of the streptococcal superantigens.