Among 325 fresh isolates of bacteria from 153 hospital patients with serious infections, 142 were susceptible to piperacillin, 129 were resistant and a further 54 exhibited a marked inoculum effect Tazobactam restored the susceptibility of resistant isolates of Staphylococcus aureus and Staphylococcus epidermidis (except methicillin-resistant isolates), Haemophilus inftuenzae, Moraxella catarrhalis, Acinetobacter spp. and Bacteroides fragilis . Among 87 Enterobacteriaceae that were resistant to piperacillin, or showed an inoculum effect, 78 (89%) were restored to susceptibility by tazobactam. Pseudomonas spp. resistant to piperacillin were mostly resistant to the combination.