This article is in part polemic but also presents quantitative evidence that the co-zymase of Harden and others is necessary to alcoholic fermentation by yeast. Alcoholic fermentation requires the enzyme system or a co-zymase, inorganic phosphate, small amounts of hexose-di-phosphate, and a suitable pH. In order to test whether a co-zymase was necessary for alcoholic fermentation, mixtures of all these constituents named above except the co-enzyme were tried. Pure glucose and phosphate mixture never fermented nor did addition of hexose-di-phosphate produce fermentation. Yeast enzyme in glucose either with or without phosphate never produced fermentation even when it stood for a long time. Acetylaldehyde or K ions added to the yeast phosphate mixture produced no fermentation. Co-enzyme made from yeast heated and killed produced fermentation, the amount produced in a given wt. of enzyme-glucose mixture being directly proportional to the amount of co-enzyme solution added. Toluol decreased the rate of fermentation. Octyl alcohol increased the rate markedly. The workers who found that co-zymase was not necessary for fermentation did not keep the zymase co-enzyme free.