The in vitro activity of the combination piperacillin/tazobactam has been compared to that of piperacillin, ticarcillin/clavulanate, cefotaxime, ceftazidime, imipenem and ciprofloxacin against 678 Enterobacteriaceae, 173 nonfermenters, 185 staphylococci, 96 E. foecalis, 217 S. pneumoniae, 202 H. influenzae and 178 M. catarrhalis isolated from hospitalized patients. Piperaeillin/tazobactam was similar in activity to third generation cephalosporins, imipenem and ciprofloxacin, and more active than piperacillin alone and ticarcillin/clavulanate.