Consecutive serum samples from patients with Staphylococcus aureus endocarditis or septicemia or non-S. aureus endocarditis and febrile nonsepticemic controls were tested for antibodies against S. aureus capsular polysaccharide (CP) types 5 and 8 by ELISA. The upper normal antibody levels were defined as the upper 99.5% confidence limits of the values from the febrile controls. All available patient isolates were tested for the presence of CP type 5 or 8 (85% of the isolates expressed either serotype), and all five patients with S. aureus endocarditis had positive antibody levels against the corresponding serotype within the first 10 days of infection. Three other endocarditis patients lacked isolates for CP testing but two of these were positive. Positive antibody levels were found in 0 of 28 septicemia patients, in 1 of 12 non-S. aureus endocarditis patients, and in 3 of 37 febrile controls. Thus, testing for anti-CP 5 or 8 antibodies, especially together with CP serotyping of the patient's isolate, seems to provide important information in the differential diagnosis of endocarditis in patients with S. aureus septicemia.