Bacterial resistance is usually a serious problem in tertiary care hospitals. The aim of this in vitro study was to evaluate the beta-lactamase inhibitor combination piperacillin/tazobactam in a hospital environment with high bacterial resistance rates and compare it with other beta-lactam agents. Three hundred and sixty-two isolates from various clinical materials were studied during the period March-August 1996. Material for culture was collected from patients of all the wards of our hospital, with the majority being from the Intensive Care Unit (45%). Pathogenic Gram-positive and Gram-negative bacteria with high resistance rates and beta-lactamase production were studied (staphylococci, enterococci, Enterobacteriaceae, Pseudomonas). Significant bacterial resistance rates were identified for ceftazidime (50% for Klebsiellae, 60% for Enterobacter spp, 60% for Proteus spp, 33% for Pseudomonas spp, 75% for Acinetobacter spp) and ciprofloxacin (33% for both Klebsiellae and Enterobacter spp, 67% for Pseudomonas spp, 50% Acinetobacter spp). Fifty percent of Enterococcus isolates were resistance to ciprofloxacin but all of them were susceptible to piperacillin/tazobactam, amoxicillin/clavulanate and imipenem. The antibacterial activity of piperacillin/tazobactam (susceptibility rates 83 to 100% for Enterobacteriaceae, 83% for Pseudomonas spp and 75% for Acinetobacter spp) was higher than that of ceftazidime, piperacillin and ciprofloxacin. Imipenem, being mostly a reserve product, showed higher activity against Acinetobacter, Klebsiella and Enterobacter species.