The metabolism of an oral dose of S-carboxymethylcysteine (SCMC) has been studied in man; the principal urinary component after 24 h was the unchanged compound (29.4-83.7%, 20 subjects). Markers variation was found between individuals; SCMC sulphoxide, S-methylcysteine, N-acetyl-S-carboxymethylcysteine, N-acetyl-S-methylcysteine with the corresponding sulphoxides and dicarboxymethylsulphide were also found as metabolites.