Data obtained from a survey of the membership of the Society for Surgery of the Alimentary Tract and the American Society of Colon and Rectal Surgeons indicated that concomitant administration of oral neomycin-erythromycin base and systemic cephalothin, together with mechanical colon cleansing, was the most popular method of colon preparation. We designed a prospective double blind clinical trial to compare administration of intravenous cephalothin, oral neomycin-erythromycin base, and the combination of both the intravenous and oral antibiotics. Intake of patients to the intravenous cephalothin group was stopped because the data indicated that this method of prophylaxis resulted in significantly higher numbers of septic complications. The incidence of wound infection was 30 per cent and the overall incidence of septic complications was 39 per cent in patients receiving only intravenous cephalothin combined with mechanical colon cleansing. The incidence of wound infection and the overall incidence of septic complications was only 6 per cent in the comparison group, and the differences are highly significant.