An enzyme-linked immunoassay (ELISA) was used to detect antibodies in human sera to Staphylococcus aureus peptidoglycan (PG) and lipoteichoic acid (LTA). All the sera from the blood donors contained IgG antibodies to both substances. Among the sera from 34 patients with bacteriologically verified, serious S. aureus infections, 71 per cent contained significantly elevated levels of anti-PG antibodies and 76 per cent of anti-LTA antibodies. Among the sera from 38 patients with suspected but not bacteriologically verified staphylococcal infections, 58 per cent contained significantly elevated levels of anti-PG antibodies and 74 per cent of anti-LTA antibodies. The levels of antibodies to PG correlated well with the levels of antibodies to LTA, but the latter occurred over a broader range in the patient sera. Elevated antibody values were, however, also found in some patients with serious, non-staphylococcal infections. The diagnostic value of PG and LTA antibodies has to be further investigated.