A substance designated as "factor Z" present in yeast extract and other extracts[long dash]not yet isolated[long dash]is capable of very materially increasing the speed of fermentation of fresh yeast without a corresponding increase in the number of cells. It is essentially different from cozymase. It is readily soluble in 94% alcohol, is not precipitated by Ba(OH)2 or lead acetate but is precipitated by lead acetate plus NaOH. It is not affected by autolysis, and it dialyses through celloidin. While boiled yeast extracts containing 2 units of cozymase accelerate the fermentation of fresh yeast by more than 100%, a purified solution of cozymase of 30 units is inactive. Conversely, fresh yeast is activated by solutions containing no cozymase. This fact is contrary to Kostychey''s assumption that cozymase activates fresh yeast. His views, therefore, have no bearing on the question of whether the fermentation by dried yeast can be ascribed to the presence of living cells. The activation of fresh yeast can neither be traced to the presence of amino acids nor to that of hexose-phosphates.