1. The inhibitory effect of glucosone on yeast fermentation was shown to the specific for the D-isomer, and exerted at some point before the breakdown of fructose-1:6-diphosphate. 2. The results of the fermentation experiments are analysed, kinetically, and glucosone shown to be a pseudo-irreversible inhibitor, of the type described by Ackermann and Potter (1949). It is shown that the effects produced by glucosone in animals can be correlated vd.th this finding. 3. The osone is shown to be phosphorylated by ATP in the presence of hexokinase. 4. It is suggested that the inhibitory effect produced by glucosone on yeast fermentation, is due to the slow dissociation of a glucosone phosphate from the hexokinase molecule. 5. The effect of hexokinase on a number of glucose analogues is reported, and indications are given of the high degree of specificity exhibited by the enzyme. 6. Methods are described for the preparation of an actively fermenting acetone-dried extract, and a maceration juice, from 'bakers' yeast. The preparation and some properties of cold-treated bakers' yeast are also described. 7. A method is described for the separation of the substrates .and products of the hexokinase reaction, on paper chromatograms.