The prophylactic effect of systemic metronidazole on infectious complications following elective colo-rectal surgery was compared with the prophylactic effect of oxytetracycline. Allocation to receive metronidazole or oxytetracycline was random. A total of 137 patients were accepted for the trial, 67 receiving metronidazole and 70 oxytetracycline. The numbers of abdominal wound infections, intra-abdominal infections, fever and deaths were few in both groups, and the differences statistically insignificant. Clinically significant side effects of the antimicrobial agents were not observed. The influence of the antimicrobial agents on colonic flora was also studied. No significant difference in the effect on colonic anaerobic flora was demonstrated, whereas the aerobic flora was significantly suppressed by oxytetracycline. It is concluded that systemic metronidazole, although acting solely on the anaerobic flora, is as effective as oxytetracycline in preventing postoperative infectious complications following elective colo-rectal surgery. The prophylactic use of metronidazole is advocated.