The effectiveness of short-term, low-dose, preoperative oral administration of neomycin and erythromycin base combined with vigorous purgation in reducing the incidence of wound infections and other septic complications of elective colon and rectal operations has been studied in a prospective, randomized, double-blind, clinical trial. One hundred and sixteen patients completed the study; all received mechanical preparation; 56 received neomycin-erythromycin base while 60 received an identical appearing placebo. The two patient groups were comparable in age distribution, clinical diagnoses, associated systemic diseases, types of operation performed and similar clinical features. The overall rate of directly related septic complications was 43 per cent in the placebo group and 9% in the group receiving neomycin and erythromycin base. The wound infection rates were 35% in placebo and 9% in antibiotic treated patients. Oral administration of neomycin and erythromycin base together with vigorous mechanical cleansing reduces the risk of septic complications after elective colo-rectal operations.