On the preoperative day before colorectal operations (n = 36), 3 g neomycine and 3 g erythromycine were administered as oral prophylactic antibiotics. No antibiotics were administered to the controls (n = 24). The number of postoperative septic complications was reduced after preoperative prophylactic antibiotics. There were two abscesses of the abdominal wound, one fecal fistula, and two anastomotic disruptions in the control group but only two abscesses of the abdominal wound in the antibiotic group. No toxic effects were observed during this trial.