A simple, rapid, and precise high-performance liquid chromatography (HPLC) method using ultrafiltration to remove plasma protein was developed to determine meropenem concentrations in human plasma in a clinical setting. Plasma was separated by centrifugation at 4 degrees C from blood collected in heparinized vacuum tubes, and meropenem was stabilized by immediately mixing the plasma with 1M 3-morpholinopropanesulfonic acid buffer (pH 7.0) (1:1). Ultrafiltration was used for plasma deproteinization. Meropenem was detected by ultraviolet absorbance at 300 nm with no interfering plasma peak. The calibration curve of meropenem in human plasma was linear from 0.05 to 100 microg/mL. Intraday and interday precision was less than 7.17% (CV), and accuracy was between 97.7% and 106.3% over 0.05 microg/mL. The limit of detection was 0.01 microg/mL. The assay has been clinically applied to a real-time therapeutic drug monitoring in pediatric patients and pharmacokinetic studies.