Abstract A technically easy, sensitive method for specific determination of cysteine was developed. It was based upon the fact that an equimolecular reaction between cysteine and noradrenochrome caused the pink color of the latter to change to yellow, while the pink color remained unchanged or merely faded in the presence of other sulfhydryl compounds. Application of the method indicated that the absence of cysteine from freshly drawn human plasma or freshly voided human urine was consistent with the rapid disappearance of cysteine added to plasma or urine.