The presence of Staphylococcus aureus producing toxic shock toxin (TST) and the absence of antibody to TST (anti-TST) in acute-phase sera are markers for toxic shock syndrome (TSS). We used radioimmunoassay methods to examine 133 acute-phase and 277 convalescent-phase serum specimens from 181 patients with TSS for anti-TST. Among confirmed menstrual cases, nine (9.5%) of 95 patients had demonstrable anti-TST in acute-phase sera obtained during the first seven days of illness; patients with probable or non-menstrual TSS had a higher prevalence of anti-TST in acute-phase sera. Five (33.3%) of 15 individuals with confirmed menstrual TSS developed anti-TST as early as seven to nine days after TSS onset; 32 (62.7%) of 51 patients had demonstrable anti-TST in sera obtained more than one year after their episode of TSS. This study demonstrates a gradual rate and low magnitude of development of anti-TST after TSS and supports the diagnostic usefulness of measuring anti-TST levels in sera from patients suspected of having TSS.