A new acid thiol, 2-mercaptopropionate, has been identified by GC-MS in urine from cystinuric patients on treatment with 2-mercaptopropionylglycine. The amount excreted was 50-300 mumol/24h and it was correlated with the oral dose of 2-mercaptopropionyl-glycine. The observation may be of clinical importance as the metabolite could be a potential hazard in oxidation of fatty acids.