Study of the bactericidal power of antibiotic combinations against 18 strains of staphylococcus aureus causing osteoarthritis showed that the combinations most often synergistic or indifferent were: rifampicin + lincomycin or synergistine, rifampicin + fusidic acid, synergistine + fusidic acid or trimethoprim-sulphamethoxazole, cephalosporins + gentamycin, rifampicin or fusidic acid + gentamycin, trimethoprim-sulphamethoxazole + rifampicin or fusidic acid. Study of the antibiotic therapy used here was based upon the inhibitory power of serum and the measurement of antibiotic concentrations in healthy bone. Combinations which are synergistic or indifferent in vitro endow the serum with a high level of bactericidal activity. Certain antagonisms (cephalosporins or penicillin M + rifampicin) found in vitro may be reflected neither clinically nor in study of the inhibitory power of the serum.