For elective alimentary tract operations in which contamination is moderate, single dose prophylaxis with piperacillin is equivalent to triple dose cefoxitin, a well established and effective regimen. Both methods failed to control infection arising in the perineal wound after abdominoperineal resection. Just as is the case with drain site infection, such infection often evolves from postoperative contamination and, indeed, is in theory and in fact unlikely to be controlled by perioperative prophylaxis. This study is among the first of several examining the issue of single dose prophylaxis and will be the harbinger of other studies from other groups examining whether or not the course of therapeutic antibiotics can be safely shortened in patients with peritoneal contamination.