The in-vitro activity of meropenem, a new parenteral carbapenem, was compared with that of imipenem, ceftazidime, cefotaxime, piperacillin, gentamicin and, where appropriate, other antibiotics against recent clinical isolates and characterized β -lactamase producers. MICs were determined by a standard agar dilution procedure and two inocula (10 4 and 10 6 cfu) were used throughout. Meropenem inhibited 90% of isolates of Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis , indole-positive Proteus spp., Enterobacter spp., Serratia marcescens and Providencia stuartii at ≤ 0.25 mg/l and was four- to 16-fold more active than imipenem against these species. Against the enteric pathogens Salmonella typhi, Shigella sonnei, Yersinia enterocolitica and Campylobacter jejuni , meropenem was four- to eight-fold more active than imipenem, inhibiting all isolates at ≤ 0.03 mg/l. Meropenem was also more active than imipenem against Haemophilus influenzae (MIC 90 0.06 mg/l) but had similar activity against the Bacteroides fragilis group (MIC 90 0.25 mg/l), against Pseudomonas aeruginosa (MIC 90 2 mg/l) and against streptococci. Imipenem was four-fold more active than meropenem against Acinetobacter spp . and two- to eightfold more active against all species of staphylococci tested. Both meropenem and imipenem were inactive against Ps. (Xanthomonas) maltophilia .