A simple, rapid, specific and sensitive high-performance liquid chromatography method has been developed for quantitation of 5-fluorouracil (5-FU) in human plasma. The method involves deproteinization of a small sample volume of plasma (150 microliters) followed by HPLC on a cation-exchange resin column, Aminex HPX-87H (300 x 7.8 mm I.D.), preceded by a similar guard cartridge with UV detection at 265 nm. This method allows a good separation of 5-FU with a retention time of 24 min and a detection limit at 25 ng/ml. The calibration curve was linear from 25 to 2000 ng/ml. The coefficient of variation was < or = 4.4% for within-day reproducibility and < or = 9.5% for day-to-day reproducibility.