SummaryThein vitroactivity of the combination piperacillin/tazobactam has been compared to that of piperacillin, ticarcillin/clavulanatc, cefotaxime, ceftazidime, imipenem and ciprofloxacin against 678Enterobacteriaceae,173 nonfermenters, 185 staphylococci, 96E. faecalis,217S. pneumoniae,202H. influenzaeand 178M. catarrhalisisolated from hospitalized patients. Piperacillin/tazobactam was similar in activity to third generation cephalosporins, imipenem and ciprofloxacin, and more active than piperacillin alone and ticarcillin/clavulanate.