The authors present a prospective trial in 34 patients undergoing elective colo-rectal surgery. Seventeen patients received 2 g of piperacillin; intravenously, 30' min before surgery, and again 5 times, postoperatively, every 6 hours. The other 17 patients received 2 g of the same drug, intravenously, every 12 hours, immediately after surgery, at least for 5 days. The first group showed postoperative wound infections, with a rate of 11.75%. In the second group 5 patients showed postoperative infection, with a rate of 29.41%. There were 3 wound infections, one urinary tract infection and two pleuropolmonitis (a patient had two infections simultaneously). No side effects were registered. The better results registered in the first group, suggest the use of piperacillin in shortterm prophylaxis. This choice is attractive because of the comparison with other antibiotics, used in a similar way, shows similar results.