Toxic-shock syndrome (TSS) is believed to be caused by a toxin produced by Staphylococcus aureus. An exotoxin has been identified that is associated with strains of S. aureus isolated from patients with TSS. Coded strains of S. aureus were tested for the presence of the exotoxin by polyacrylamide gel isoelectric focusing. Sixty isolates of S. aureus were tested; 28 (100%) of 28 isolates from patients with TSS but only five (16%) of 32 control isolates produced the toxin (P much less than 0.001). This protein exotoxin, which was purified by differential precipitation with ethanol and thin-layer isoelectric focusing, had an isoelectric point of 7.2. When tested by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the toxin migrated as a homogeneous protein with a molecular weight of 22,000. The exotoxin produced fever in rabbits and enhanced susceptibility to lethal shock caused by endotoxin. Other biologic properties of the exotoxin included lymphocyte mitogenicity and the capacity to suppress synthesis of IgM antibody to sheep erythrocytes.