A variety of methods have been used for the determination of sulfhydryl groups in compounds of biological importance, but only some of these are applicable to the estimation of sulfhydryl groups in proteins. It is the purpose of this paper to show that the amperometric procedure described for the determination of mercaptans by Kolthoff and Harris can be applied to the determination of sulfhydryl groups in cysteine and glutathione, as well as in certain proteins.