In a prospective, randomized study on surgical prophylaxis in elective gastrointestinal operations, 145 adults were allocated to 1 of 3 groups. Those in group 1 received 1 preoperative dose of 200 mg of doxycycline and 2 g of tinidazole. Those in group 2 were given 3 doses of 80 mg of gentamicin and 600 mg of clindamycin with 8 h intervals beginning 1 h preoperatively. Group 3 patients served as controls and obtained no antimicrobial agent. The prophylactic regimens reduced the infectious complications, as evidenced by fewer infections, less total antibiotic dosage and shorter hospitalization. In the prophylactically treated groups, anaerobic infections were less frequent than in those in the control group.