Ninety patients were included in this prospective randomized trial. Each required electric colorectal surgery and was prepared for operation with oral preoperative antibiotic therapy, systemic peroperative therapy, or by a combination of both. The number of each type of septic postoperative complication and their total did not differ between the group treated by oral antibiotics prior to operation and the group treated peroperatively with systemic antibiotic therapy. The total number of septic complications (wall abscesses, fistulas, subdiaphragmatic abscesses, septicemia, peritonitis), however, was significantly less (P less than 0.05) in the group treated by both preoperative oral antibiotics and peroperative systemic antibiotic therapy (3.3 per cent) than in either groups treated only orally preoperatively (30 per cent) or by systemic antibiotic therapy during the operation (23 per cent). The combination of oral antibiotic therapy prior to operation and of systemic peroperative antibiotic therapy, therefore, presents the most effective prophylactic effectiveness.