The determination of S‐carboxymethyl‐(R)‐cysteine (SCMC) in human plasma during extended bioequivalence studies demands a rapid, accurate and selective assay technique. A liquid chromatographic/mass spectrometric method was developed which involves rough protein precipitation followed by high‐performance liquid chromatographic separation with an ion‐exchange column and atmospheric pressure ionization (API) mass spectrometric detection, with the instrument operating with electrospray ionization (ESI) and in the selected‐ion monitoring mode. The drug and the internal standard S‐[(R)‐1‐carboxyethyl]‐(R)‐cysteine (SCEC) are detected by focusing the first quadrupole of the triple stage system on MH+ ions, thus permitting elimination of endogenous interfering substances and allowing a detection limit of 0.05 μg ml‐1. The chromatographic run time is 16 min and the method has sufficient sensitivity, precision, accuracy and selectivity for routine analyses of clinical plasma samples containing SCMC at concentrations in the range 0.2–20 μg ml‐1. In summary, this LC/MS‐based assay of SCMC demonstrates advantages of easy sample preparation, low limit of quantification (200 ng per ml of human plasma) without any derivatization step, high specificity and rapid sample analysis with an overall throughput of more than 60 analyses per day. © 1997 by John Wiley & Sons, Ltd.