A prospective randomized single blind controlled clinical trial was undertaken to compare prophylactic therapy using a systemic antibiotic active against both aerobic and anaerobic bacteria with an oral antibiotic agent active only against anaerobic bacteria in elective colorectal surgery. One hundred and thirty-one patients received ticarcillin and 130 received tinidazole. The wound infection rate was 8% in those patients receiving ticarcillin prophylaxis and 20% in those receiving tinidazole (P < 0.05). Multivariate analysis of the factors affecting wound infection rate showed that there were three independent factors that reached statistical significance: the prophylactic antibiotic used; the type of hospital (public or private) in which the operation was performed, and the presence of a stoma at operation. The wound infection rate in those patients receiving tinidazole prophylaxis was more than twice that reported previously by the authors. The mortality in patients receiving ticarcillin prophylaxis was 1.5% compared to 9.2% in those receiving tinidazole prophylaxis (P < 0.05). The clinical anastomotic leakage rate was similar in each antibiotic prophylactic group, 8.6% in those receiving ticarcillin and 7.3% in those receiving tinidazole.