An unidentified ninhydrin-positive substance of an acidic nature was detected in the plasma of uraemic patients. This substance was isolated from haemodialysate by ion-exchange chromatography and gel filtration, and identified as a sulphur-containing amino acid: N-monoacetylcystine. 2. The quantitative determination of sulphur amino acids in plasma revealed that the plasma levels of cysteic acid, homocysteic acid, taurine, cystine and cystathionine as well as N-monoacetylcystine in uraemic patients were markedly higher than in normal subjects (P less than 0.001 for each). However, the plasma levels of methionine in uraemic patients were within normal limits.
