A comparison was made of two methods of prophylactic antibiotic therapy against the septic complications of colonic and rectal surgery: --preoperative oral antibiotics associated with peroperative systemic antibiotics; --peroperative systemic antibiotics only, continued for 24 hours after surgery. Both types of antibiotic therapy were of short duration and were designed to cover aerobic and anaerobic organisms. Two groups of 30 patients were selected at random. They were homogeneous. The septic complication rate was 10% in the oral plus systemic and 24% in the systemic group. It is felt that the combination of oral and systemic antibiotics remains preferable, in particular bearing in mind the efficacy of oral metronidazole.