The role of antibiotic prophylaxis in operations upon the colon and rectum is now well recognized. In this study, the efficacy of cefotetan, a broad spectrum cephamycin antibiotic, given alone (n = 315), is compared with that of the combination of cefotetan and metronidazole (n = 300). The groups were well matched for age, sex, pathologic features and operations performed. There was no statistical difference in the incidence of operation-related infection between the two groups. We conclude that cefotetan given alone is as effective as the combination of cefotetan and metronidazole for prophylaxis against operation-related infection in elective colorectal operations.