The in vitro antimicrobial spectrum of piperacillin-tazobactam is described and the results of clinical trials are summarized. Injectable products combining a beta-lactam antibiotic with a beta-lactamase inhibitor have a broad spectrum of activity, principally influenced by the potency of the beta-lactam. Piperacillin-tazobactam can be used for the treatment of infections caused by gram-negative, gram-positive, aerobic, and anaerobic bacteria. Appropriate indications include infections caused by mixed flora, such as gynecologic, intra-abdominal, and surgical wound infections. Other infections that appear responsive include those associated with human and animal bites, ischemic or diabetic foot infections, infected ischemic or pressure ulcers, and lower-respiratory-tract infections. Piperacillin-tazobactam has been reported to be superior to ticarcillin-clavulanate in community-acquired lower-respiratory-tract infections and superior to imipenem-cilastatin (at 1.5 g/day of imipenem) in intra-abdominal infection.