The in vitro activity of ertapenem (MK-0826), a new carbapenem, was studied against 389 aerobic microorganisms (187 Enterobacteriaceae, 15 Aeromonas spp., 42 Staphylococcus spp., 43 Streptococcus spp., 15 Enterococcus spp., 30 Haemophilus spp., 15 Moraxella catarrhalis, 12 Neisseria gonorrhoeae, 15 Pseudomonas aeruginosa and 15 Acinetobacter spp.) and 54 anaerobic isolates (15 Clostridium spp., 12 Peptostreptococcus spp. and 27 fragilis group Bacteroides recovered from four Spanish hospitals. Ertapenem activity was compared with that of imipenem, piperacillin-tazobactam, cefoxitin, ceftriaxone, ceftazidime, cefepime, and norfloxacin. Ertapenem was the most active antibiotic against Enterobacteriaceae (MIC(90) < or =0.5 mg/l) particularly in the case of broad-spectrum, extended-spectrum and chromosomally encoded AmpC betalactamase-producing strains. Ertapenem exhibited less activity, even lower than that of imipenem, against P. aeruginosa, Acinetobacter spp. and enterococci (MIC(90) > or =16 mg/l). Ertapenem was active against methicillin-susceptible S. aureus and coagulase-negative staphylococci, beta-haemolytic streptococci, and Streptococcus pneumoniae (MIC(90) < or =1 mg/l). In the case of Haemophilus spp., M. catarrhalis and N. gonorrhoeae, ertapenem, with a MIC(90) < or =0.06 mg/l resulted the most active antibiotic tested. When considering the anaerobes, ertapenem displayed a broad spectrum of activity, similar to that of imipenem, against Clostridium spp. (MIC(90) 2 mg/l) and was slightly less active against Bacteroides fragilis (MIC(90) 0.5 mg/l). Both carbapenems were the most active among the tested compounds. Due to its activity against almost all pathogens studied, ertapenem appears to be an option for the treatment of mixed bacterial infections.