The efficacy and safety of piperacillin plus tazobactam and of imipenem plus cilastatin were compared in an open randomized multicentre study. In the piperacillin/tazobactam group, 40 men and 29 women (mean age 53 years, range 18-92) received 4 g piperacillin with 500 mg tazobactam every 8 h; in the imipenem/cilastatin group 40 men and 25 women (mean age 54 years, range 16-91) received 500 mg imipenem with 500 mg cilastatin, also every 8 h. Antibiotics were administered intravenously for at least three days and for not more than 14 days. Infections were verified by culture of material obtained at laparotomy or by puncture of an abscess before the start of therapy. Fifty of 55 evaluable patients in the piperacillin/tazobactam group (91%) and 40 of 58 evaluable patients in the imipenem/cilastatin group (69%) were clinically cured. Four relapses or failures were recorded in the piperacillin/tazobactam group and 18 in the imipenem/cilastatin group (P < 0.005). Eradication of the microorganisms isolated were similar in both groups. Adverse reactions in both groups were mild and dominated by nausea and diarrhoea. At the dosage used piperacillin/tazobactam was as safe as, and statistically more effective than, imipenem/cilastatin in the treatment of intra-abdominal infections caused by sensitive organisms.