Ertapenem, a novel carbapenem, is approved for the treatment of mild to severe intra-abdominal infections (IAIs), although its in vivo concentrations in peritoneal fluid are unknown. The purpose of this study was to determine the peritoneal fluid concentration of ertapenem after a single 1 g intravenous dose. After informed consent, 21 patients (9 females and 12 males; mean+/-standard deviation (S.D.) age 50.2+/-17.7 years) requiring intra-abdominal surgery were enrolled. Plasma and peritoneal fluid samples were taken at fixed times during surgery. Drug concentrations were determined by high-performance liquid chromatography (HPLC) with ultraviolet detection. Mean+/-S.D. ertapenem peritoneal fluid concentrations were 64.3+/-23.4 mg/L at 1h and 31.3+/-26.5 mg/L at 3 h after administration. The mean tissue/plasma ratio ranged from 46.7% to 83.1%. The mean peritoneal fluid concentrations were well above the MIC(90) (minimum inhibitory concentration for 90% of the organisms) for susceptible bacteria found in IAIs, especially Escherichia coli, viridans streptococci, Enterobacteriaceae, Klebsiella spp. and Bacteroides fragilis, during the entire sampling time. These pharmacokinetic results support the assumption that ertapenem might be suitable for the treatment of IAIs.