Ertapenem is an important newer broad-spectrum carbapenem antibiotic covering various infections caused by common gram-positive and -negative aerobes and anaerobes. Due to its physicochemical peculiarities, pharmacokinetic data of other carbapenems are of limited value in predicting ertapenem distribution into particular compartments of the body. This raises demand for detailed pharmacokinetic studies and, as a consequence, rapid and specific ways of analysis. The HPLC assays for the quantification of ertapenem in biological matrices reported so far are based on columns of 4.6mm I.D. and involve pre-concentration by use of column-switching. However, automated column-switching technique is not standard equipment with all analytical laboratories. Furthermore, signal-to-noise ratios are likely not to be sufficient for quantification of specimens of low concentration. Therefore, a new HPLC/UV method based on narrow-bore column design using sample pre-cleaning by liquid-liquid extraction has been developed. The assay is rapid for specimen concentrations > or =1 mg/l and is easily tuned to achieve low quantification limits at high chromatographic resolution for lower concentrated samples. The method has been successfully applied to plasma, serum, lung tissue or cell homogenates, and broncho-alveolar lavage fluid with lower limits of quantification of 40 and 20 microg/l, respectively. It was also used for the pharmacokinetic monitoring of ertapenem in humans.