In a prospective, randomized trial, prophylactic use of topical antibiotics in addition to systemic prophylaxis was studied in patients undergoing abdominoperineal amputation of the rectum. All patients received gentamicin 80 mg and metronidazole 500 mg intravenously at induction of anesthesia, followed by the same dose 8 hourly for 48 hours. In accordance with the randomization, half of the patients were additionally given gentamicin 160 mg + metronidazole 400 mg topically into the perineal wound at closure. Perineal wound infection appeared in 19 of the 41 patients who received both systemic and topical prophylaxis, and in 18 of the 38 with only systemic antibiotics. Cell-mediated immunity was preoperatively assessed with a skin test (Multitest) in all but three patients. Impairment of cell-mediated immunity was associated with significantly heightened rate of wound infection, and these patients did not benefit from topical antibiotics.