612 (93.8%) of 65 Staphylococcus aureus strains isolated from 65 patients with toxic-shock syndrome (TSS) produced an enterotoxin-like protein, tentatively identified as staphylococcal enterotoxin F (SEF). One of the other strains produced staphylococcal enterotoxin B and another exterotoxin C. In two blind studies all 34 TSS-associated S. aureau strains examined and 3 (11.5%) of 26 control S. aureau strains produced SEF. 2 of the latter strains were isolated from the vaginas of women who had no history of TSS. SEF was purified, and specific antibodies to it were prepared. Only 4 (4.6%) of 87 S. aureau strains from other sources were found to produce SEF. 5 (17.2%) of 29 TSS patients whose acute sera were available had anti-SEF antibody present in titres of greater than or equal to 1:100 as determined by radioimmunoassay, compared with 44 (78.6%) of 56 controls--demonstrating a greater serosusceptibility of TSS patients to SEF. It is suggested that staphylococcal enterotoxin, particularly SEF, may be a cause of the signs and symptoms of TSS.