The efficacy of ticarcillin in the prevention of post-cesarean section endomyometritis was studied in 259 women randomly given either the antibiotic or a placebo. The ticarcillin group received 6 g intravenously immediately after delivery. Then 22 of those patients were also given a second 3-g dose 6 to 8 hours after delivery. Among the 139 patients who received ticarcillin, endomyometritis developed in 44 (32%), as it did in 66 of 120 patients (55%) in the placebo group. These differences are highly significant (P = .002). Of the 259 patients in the study, 238 could be classified as to risk for endomyometritis developing. Among the 124 high-risk patients, 52 received a placebo and endomyometritis developed in 71%. Only 26 of the 72 high-risk patients who received ticarcillin (36%) were so infected. Among the low-risk patients, endomyometritis developed in 10 of 54 patients (18.5%) who received prophylaxis and in 24 of 60 patients who received placebo (40%). Ticarcillin appears to be effective in reducing the incidence of post-cesarean section endomyometritis in patients at high risk and in those at low risk. The number of pelvic examinations during labor was the most important single factor in the development of endomyometritis.