Twenty-nine British and Irish hospitals each collected up to 300 bacterial isolates from in-patients. The organisms were identified by an appropriate API system or, for staphylococci, by their Gram and coagulase reactions. Disc susceptibility tests were performed. Isolates that gave zones < or = 25 mm to piperacillin/tazobactam (75 micrograms + 10 micrograms) discs were sent to a central laboratory for re-examination and determination of MIC, together with a sample of the more susceptible organisms. Results were evaluated for 6724 isolates. Over 95% of the isolates of Escherichia coli, klebsiellae, Proteus mirabilis, Pseudomonas aeruginosa, Haemophilus, Moraxella and Bacteriodes, spp. streptococci, pneumococci and Enterococcus faecalis were susceptible to piperacillin/tazobactam (defined as giving a zone > or = 22 mm to a 75 micrograms + 10 micrograms disc), as were 86% of Acinetobacter spp. and 82% of the Citrobacter, Enterobacter, Morganella and Serratia group. Tazobactam particularly extended the activity of piperacillin against E. coli isolates (96% susceptible cf. 61% to piperacillin alone) klebsiellae (95% cf. 70%), P. mirabilis (99% cf. 86%), and Acinetobacter spp. (86% cf. 53%). Occasional (18%) resistance in Enterobacter, Serratia and Citrobacger spp. was probably caused by stable depression of Class I beta-lactamases, which are inhibited poorly by tazobactam. High resistance frequencies (> 25%) were found for Enterococcus faecium and Xanthomonas maltophilia. Tazobactam potentiated piperacillin against beta-lactamase-producing methicillin-susceptible Staphylococcus aureus, but the mode inhibition zone of piperacillin/tazobactam discs was only 26 mm, compared to 38 mm for beta-lactamase-negative isolates. Nevertheless, fewer than 5% of the enzyme producers appeared resistant to 8 + 4 mg/L piperacillin/tazobactam in MIC tests. Similar behaviour was noted for coagulase-negative staphylococci. Amongst the eleven comparator drugs, ceftazidime, gentamicin and ciprofloxacin were as active as piperacillin/tazobactam against most enterobacteria. However, Acinetobacter and Bacteroides spp. and enterococci were resistant to ceftazidime, and Bacteroides spp., enterococci, pneumococci and other streptococci were inherently resistant to ciprofloxacin and gentamicin. Cefuroxime, ampicillin and co-amoxiclav had narrower spectra. Only imipenem showed a consistently wider spectrum and lower frequency of resistance than piperacillin/tazobactam.