In this study, a new analytical method for erdosteine (ERD) in plasma based on high-performance liquid chromatography and a fluorimetric detector, is presented. Precolumn derivatization of ERD with 4-bromomethyl-7-methoxy coumarin (BrMmC) and dibenzo-18-crown-6-ether as a reaction catalyst led to the production of a fluorescent compound. ERD was monitored by fluorescence with an excitation wavelength lambda(ext.) = 325 nm and emission wavelength lambda(em.) = 390 nm. Optimum reaction conditions were carefully studied and optimized. A chromatographic procedure was performed using a C18 column of 150 x 4.6 mm and 3 mu m particle size and a mobile phase consisting of methanol:acetonitrile:water (30:30:40, v/v/v) under a flow rate of 0.5 ml min(-1). A calibration plot was established covering analyte concentration range 0.2-3.0 mu g ml(-1); the detection limit was 0.015 mu g ml(-1) and quantification limit was 0.05 mu g ml(-1). Mean recovery was 87.33% and relative standard deviation was calculated to be less than 4.4%. The developed method was successfully used to determine pharmacokinetic preparations of ERD subsequent to administration of a 900 mg dose capsule to a healthy 40-year-old woman volunteer.