Fusidic acid and cloxacillin were studied in patients who underwent joint aspiration for noninfectious disorders. Nine patients were given oral 500 mg fusidic acid tid for 72 h, the last dose being given 4, 8 or 12 h before the joint aspiration. Cloxacillin was administered in a single 2 g iv dose to 9 patients, 0.5, 4 or 8 h before the aspiration. Bactericidal activity was determined against five isolates each of methicillin-susceptible and methicillin-resistant Staphylococcus aureus. Satisfactory activity (> or = 1:3) was detected in the serum in patients who received fusidic acid, while in the synovial fluids titres reflected borderline effectiveness (c. 1:2). Despite drug concentrations and excellent MICs, fusidic acid demonstrated markedly lower inhibitory and bactericidal activity against S. aureus than did cloxacillin.