Twenty courses of fusidic acid were given to 16 patients with antibiotic-associated colitis caused by Clostridium difficile. Fusidic acid was given in a dose of 0.5-1.5 g daily for seven to 21 days. Diarrhoea disappeared rapidly. Clinical relapse occurred after five courses and once when the patient was still on treatment. Clinical cure with persistence or reappearance of toxin occurred in four further patients. Nineteen courses of metronidazole were given to 19 patients who experienced six failures or relapses. Seven courses of vancomycin were given to five patients, three of whom had had relapse. Five patients healed without treatment. The relapses occurred only in old and prostrated patients. They often recurred several times in the same patient. 0.5 g of fusidic acid daily appears to be as effective as vancomycin and metronidazole for the treatment of C. difficile-induced colitis.