Toxic-shock-syndrome toxin I (TSSTI), an exotoxin produced by certain strains of Staphylococcus aureus, has been closely associated with the pathogenesis of toxic shock syndrome. Outside the context of its staphylococcal host, TSSTI may offer therapeutic uses. We report here a strategy for high-level expression and simplified purification of TSSTI. We have subcloned the coding region for TSSTI into a vector containing an inducible T7 promoter sequence and expressed the protein in an Escherichia coli host strain. The recombinant TSSTI protein contained ten sequential histidine residues (Histag) at its N-terminus, which enabled its efficient purification using nickel-agarose-affinity resin. Histag-TSSTI (H-TSSTI) was further purified to homogeneity using a size-exclusion column. By this system, 80 mg of highly purified H-TSSTI can be consistently obtained per litre of culture in under 3 days. H-TSSTI retained biological activity and was unaffected by the presence of the Histag, as measured in lymphocyte proliferation assays.