That yeast possesses powerful reducing properties has long been known and many investigations have been made as to the relation of these properties to the process of alcoholic fermentation. Thus Hahn (Buchner, E. and H., and Hahn, 1903, p. 343) found that the power of reducing methylene blue was possessed both by yeast and zymin and on the whole ran parallel to the fermenting power in the process of alcoholic fermentation. The intervention of a reducing enzyme was suggested by Grüss [1904, 1908, 1, 2] and was supported by Palladin [1908]. The latter observed that zymin which reduces sodium selenite and methylene blue in absence of sugar almost ceases to do so in presence of a fermentable sugar, and concluded that the great diminution of reduction during fermentation was due to the fact that the reducing enzyme was largely combined with a different substrate arising from the sugar, the reduction of which was necessary for alcoholic fermentation. Grüss, however, found that with living yeast the reduction is greatly increased in presence of a fermentable sugar, while Harden and Norris, R. V. [1914] confirmed the observation of Grüss, but found that the reducing power of zymin is not seriously affected by the presence of a fermentable sugar in concentration less then 20 grams per 100 c.c., whilst its fermenting power for glucose is inhibited by 1 per cent. sodium selenite. Hence Palladin's conclusion cannot be regarded as proved. Interesting attempts have been made by Kostytscheff and later by Lvoff to obtain evidence of the participation of a reductase in alcoholic fermentation by adding some substance which would be capable either of taking up hydrogen and thus preventing the reduction of the acetaldehyde or of converting the aldehyde into some compound less liable to reduction.