A method of estimation by high-performance liquid chromatography of ketoprofen in plasma is proposed. This method is rapid, simple, and very sensitive: it allows concentrations of 2 ng/ml to be determined. The internal standard is (benzoyl-4-phenyl)-2-butyric acid. The drug is extracted from a biological fluid in ether, which is then evaporated; the residue is taken up in chromatographic eluant (acetonitrile/phosphate buffer pH 7), analysed by reverse-phase chromatography and detected by spectrophotometry at 256 nm. In spite of the very strong protein binding of this drug, this method allowed the determination of its free concentration in plasma.