The novel method now described, forS-carboxymethylcysteine in plasma at levels that may be lower than 20 μg/ml in therapeutic investigations, entails conversion into the corresponding phenylthiohydantoin (PTH) derivative, so as to confer ultraviolet detectability in a final separation step entailing reverse-phase HPLC. The conditions adopted take account of possible losses of the compound, associated with capricious water-solubility, of stability problems unexpectedly encountered with its PTH derivative, and of the risk of interferences from plasma constituents.