During a one year and six month period, 137 patients undergoing elective colorectal surgery for carcinoma were randomly allocated to three groups. Patients in group A received oral tobramycin and metronidazole for three days prior to surgery. Patients in groups B and C received systemic antibiotic, a second generation cephem cefmetazole, every 3 hours during surgery. Patients in group C also received a luminal preparation of tobramycin during surgery. All patients received mechanical bowel preparation in the same manner. The incidence of postoperative wound sepsis was 10.9% in group A, 9.8% in group B, and 10.0% in group C. There were no significant differences in postoperative wound sepsis among the three groups. The oral antibiotic regimen induced a greater change in intestinal flora and was associated with more frequent postoperative diarrhea. Although the preoperative bowel preparation taken for patients in group A could not sufficiently reduce the intestinal bacterial count, systemic antibiotic prophylaxis was simple and cost-effective. There was no additional advantage in combining the systemic and luminal antibiotic preparations. Therefore, for elective colorectal surgery, we recommend intravenous infusion of second generation cephem cephmetazole with mechanical bowel preparation.