A sensitive isocratic high-performance liquid chromatographic (HPLC) method for the measurement of melphalan in plasma is presented. It requires an extraction step using columns of XAD-2 resin before injecting the clarified methanol eluate directly into the HPLC system. The HPLC system uses an isocratic mobile phase containing an ion-pair reagent, and a sensitive fixed-wavelength (254 nm) monitor with a noise specification of less than 2 . 10(-5) absorbance units peak to peak. The concentration of melphalan was followed in a patient with multiple myeloma on day 1 and day 4 of a four-day course of the drug. Little difference was detected between the two curves with terminal half-lives of 71 and 68 min respectively and areas under the curve of 1.08 and 1.15 min . microgram/ml . (mg dose)-1.