To study adverse reactions associated with intravenous administration of fusidic acid 6 patients were treated with fusidic acid intravenously in association with a major large bowl operation, and 9 patients were treated in the same way because of staphylococcal infections. The main adverse reaction was thrombophlebitis, which occurred in as many as 12 of 14 patients who were treated for 2 days or longer. Three surgical patients developed postoperative hyperbilirubinaemia, but studies of liver function before and during treatment in 6 of the patients with staphylococcal disease revealed no adverse liver reactions. Intravenous administration of fusidic acid into a peripheral vein for 24 h or more involves an extremely high risk of developing thrombophlebitis.