To compare the efficiency of antibioprophylaxis by cefazolin sodium or cefotaxime sodium, 3137 consecutive patients undergoing abdominal surgery were included in a prospective, randomized, controlled, multicenter study. The patients were divided into four strata, according to the degree of contamination during the operation and the risk factors. Within each stratum, the patients were randomized into three groups of treatment: (1) cefazolin, (2) cefotaxime, and (3) nontreatment (control). Antibiotics were administered perioperatively in three intravenous doses of 1 g at eight-hour intervals. Patients undergoing colon surgery or with peritonitis at the time of the operation were excluded from the study. The wound abscess rate was significantly lower in the treated groups than in the control group, except in stratum 3 (contaminated surgery). The percentage of postoperative peritonitis was twice as low in the treated groups as in the control group. There was no difference between the groups receiving cefazolin or cefotaxime. The patients in the treated groups received significantly less postoperative antibiotics than the patients in the control group. In terms of cost, antibioprophylaxis by cefazolin seems to be warranted in all operations with a low anaerobic contamination.