A randomized controlled trial was performed to compare the effectiveness of ticarcillin/clavulanic acid to metronidazole/netilmicin in preventing postoperative infections after elective colorectal surgery. Ninety-two patients were randomly allocated to receive three doses of ticarcillin, 3 g, with clavulanic acid, 100 mg every 8 hours, or three doses of metronidazole, 500 mg, and netilmicin, 80 mg intravenously every 8 hours. All patients received a mechanical bowel preparation with 3 L of Go-lytely solution. There were no operative deaths. Eight patients had wound infections--three in the ticarcillin/clavulanic acid group and five in the metronidazole/netilmicin group. One patient in the ticarcillin/clavulanic acid group had an intra-abdominal abscess. There were two anastomotic leaks, one in each group. Ticarcillin/clavulanic acid and metronidazole/netilmicin appear to be equally effective in preventing postoperative infections after elective colorectal surgery.