The initial phase of fermentation with whole yeast shows the onset of an apparent formation of lactic acid, which can be markedly increased by watery extracts of Aspergillus niger. On transition to the stationary phase this lactic acid decreases to an amount which remains constant throughout the fermentation. By forming baryum salts this lactic acid can be separated into two fractions. One of these, the precipitate which is insoluble in alcohol, is to be regarded as a triose-phosphoric acid, according to its properties. This seems to prove that the decomposition of sugar to alcohol by intact living yeast also proceeds by way of triose phosphoric acid as intermediate, just as in the case of cell-free maceration juice. It is rather improbable that the subsequent reactions would follow another route than has been indicated in the scheme of alcoholic fermentation. The formation of this triose phosphoric acid has not yet been “synchronized” with the following reactions during the initial phase of fermentation, so the preceding formation of this triose phosphoric acid can be demonstrated analytically. In the stationary phase, however, this intermediate is not present in larger amount, as is to be expected. These experiments also reveal the origin of the small amounts of lactic acid which are formed during each yeast fermentation. This origin is to be found in the initial phase and the amount of lactic acid gradually increases when the fermentation proceeds until a constant value is attained which is mostly 30–60 mg/100 ml wort for the wine yeasts investigated. It is not yet clear whether this lactic acid originates from a conversion of the metabolism of resting yeast to that of budding yeast, or whether it must be regarded to be a product of the preceding formation of triose phosphoric acid. In the latter case it would of course not have been formed by alkaline saponification. It could however arise from the spontaneous decomposition of triose phosphate to methylglyoxal. The latter would then be converted into lactic acid by methylglyoxalase. This would be the first time that a function is appointed to this enzyme in the yeast (See also K. Lohmann 8).