A single 5-gm dose of mezlocillin or a placebo was administered intravenously 30 minutes before surgery to patients undergoing emergency cesarean section. The assignment of drug or placebo was randomized. Postoperative morbidity occurred in 62.5% of patients receiving placebo and in 18.4% of those receiving mezlocillin (P less than 0.001). The incidences of febrile morbidity, endometritis, and urinary tract infection were significantly lower in the group given mezlocillin. Other benefits of antibiotic prophylaxis included a shorter hospital stay and no serious infections in the group given mezlocillin.