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IN an earlier paper [Herbert and Groen, 1929] it was shown that the discrepancies between the results of blood-sugar methods were due to non-glucose reducing substances present mainly in corpuscles. It appeared that ferric hydroxide filtrates and zinc hydroxide filtrates contained little or none of the non-glucose material, and that therefore the original MacLean method and the original Hagedorn-Jensen method gave figures close to the true sugar value. The non-sugar material was shown to be present in tungstic acid filtrates, and to have a marked effect on all the analytical methods applied to tungstic acid filtrates except that of Benedict [1928]; the Hagedorn-Jensen method, applied to tungstic acid filtrates, was affected most of all. It was suggested that the principal non-glucose substance responsible was glutathione.In the present investigation the effect of pure reduced glutathione on bloodsugar methods has been studied. The work falls into two sections; (1) a study of the effect of glutathione on the analytical methods, when no treatment corresponding to protein precipitation is used, and (2) a study of the behaviour of glutathione in various methods of protein precipitation, using mixtures of plasma and glutathione.Two different samples of glutathione were employed, both of which were received from Sir Frederick Hopkins's laboratories. The first, received in December 1928, was a deliquescent substance, the solutions of which had a smell resembling that of mint. The second sample was received in July 1929, and consisted of the pure tripeptide of glycine, glutamic acid, and cysteine, prepared by Hopkins's new method [1929].The following blood-sugar methods have been used.(1) The Folin-Wu method [1920].(2) The Shaffer-Hartmann method as modified by Somogyi [1926].(3) The Hagedorn-Jensen method [1923].(4) The Benedict method [1928].