An assay has been developed and validated for the routine monitoring of mivacurium in plasma. It consists of liquid-liquid extraction with dichloromethane and high-performance liquid chromatography with fluorometric detection (excitation and emission wavelengths 220 nm and 320 nm, respectively). A Spherisorb C, 5 microns column and a mobile phase containing acetonitrile, KH2PO4 and methanol are used. At a flow-rate of 1 ml/min, a concentration gradient is applied. The detection limit is approximately 1 ng/ml in plasma. For the separation of steroisomeres, the Spherisorb SCX 10 microns column and acetonitrile-Na2SO4 as a mobile phase can be used. The assay shows good linearity over the range 1-1000 ng/ml. The accuracy and precision allows the utilisation in clinical pharmacokinetic studies.