The kinetics of Il-6 serum concentrations were analyzed in 17 patients with a culture verified Staphylococcus aureus septicemia. The analyses were performed using an antigen capture immunoassay. All patients had elevated IL-6 serum concentrations on admission. In most cases a subsequent rapid decrease to low levels was registered within 10 days. The IL-6 serum concentrations reflected the clinical course. In sera sampled on admission a relationship was found between IL-6 serum concentrations and levels of C-reactive protein.