In a prospective study of 65 patients with S. aureus septicemia, the clinical value of measuring serum IL-6 and lactoferrin levels was assessed and compared with CRP levels and WBC count. 20/65 (31%) patients had a CRP value < or = 100 mg/l on admission and 10 (50%) and 11 (55%) of these had serum levels of IL-6 > 100 pg/ml or lactoferrin > 2.0 mg/l, respectively. 41/64 (64%) patients had a WBC count < or = 15.0 x 10(9)/l and the corresponding figures for increased IL-6 and lactoferrin values were 29 (71%) and 21 (51%) patients, respectively. The high concentrations of IL-6 and lactoferrin on admission decreased rapidly during the hospital stay, better reflecting the clinical course than CRP and WBC count. Patients with endocarditis showed higher IL-6 levels and body temperatures both on admission and during the first days of hospitalization compared with patients without endocarditis.