The effect of antibiotic colon preparation on the tensile strength and morphologic appearance of colonic anastomoses has been investigated in dogs. Preparation with preoperative kanamycin did offer a slight benefit over mechanical bowel cleansing. The addition of erythromycin and kanamycin to the preoperative bowel preparation significantly (p less than 0.001) improved the mean tensile strengh of the healing colonic anastomosis. Continuation of erythromycin and kanamycin for one week postoperatively almost doubled the mean tensile strength of the healing anastomosis. Continuation of erythromycin without kanamycin in the postoperative period provided little protection over the preoperative administration of kanamycin and erythromycin. Histologic examination showed that erythromycin and kanamycin given during the pre- and postoperative period changed the mode of healing from that of secondary intention to healing by first intention. The findings support the concept that soilage of the peritoneal cavity at the time of elective colon surgery is not as important as leakage caused by poor anastomotic healing. Antibiotic bowel preparation with kanamycin and erythromycin improves the healing of colonic anastomoses.