The recent development of a rapid method for the estimation of foreign sugars in the blood in the presence of dextrose1 throws new light on the chemical nature of the reducing substances circulating in the blood stream under normal and pathologic conditions. The methods that have previously been used for this determination fall into three classes: (1) physical methods (polariscopy, etc.), (2) chemical methods, dependent on the rate of reaction of the various sugars under fixed conditions, and (3) biologic methods which depend on the selective action of living organisms. The chemical reactions have not been sufficiently specific. It has been very difficult to get the sugar solution pure enough for physical measurements because of the variety and great number of other optically active substances always present in the blood. The application of yeast fermentation, which theoretically should remove all the dextrose, has resulted in entirely conflicting data. Neuberg2 attributed