A randomized, double-blind, placebo-controlled study was done of a short course of cefamandole administered intravenously after cord clamping as prophylaxis in women undergoing primary nonelective cesarean section in a community hospital. Duration of labor equal to or more than 14 hours was the only significant risk factor between patients who had postoperative infectious morbidity and those who had none. Four of 43 patients (9.3%) who received cefamandole, as opposed to 13 of 47 (27.7%) who received the placebo, developed infections (p less than 0.05). This difference was reflected totally in the difference in endomyometritis development between the groups. The use of intraoperative culturing predicted infection in 4 of 13 patients in the placebo group who developed infections postoperatively. No adverse side effects were noted, and there were no cases of delayed serious infection.