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A high-performance liquid chromatographic method has been developed for the determination of the mucolytic drug S-carboxymethyl-L-cysteine (SCMC) and its major metabolites S-methyl-L-cysteine (SMC), SCMC-sulphoxide and SMC-sulphoxide. Pre-column derivatisation with o-phthalaldehyde ethanethiol and electrochemical detection were employed. The mobile phase was methanolphosphate buffer of various compositions. SCMC and its sulphoxide were determined simultaneously but, for optimum resolution, they were determined separately using a different mobile phase composition for each. The method, which was convenient and rapid, allowed the resolution of the sulphoxide diastereoisomers. Good recoveries of SCMC and its metabolites were obtained from spiked urine samples. All four compounds were detected in the urine of human volunteers during a 24-h period following a single dose of the drug. The major constituent recovered was unchanged SCMC.