A very important characteristic of the action of yeast on the amino-acids is that the two stereo-isomerides of these optically active compounds are fermented at different rates. When inactive, racemic leucine is treated with yeast and sugar, the naturally occurring component, the /l/-leucine, is more rapidly attacked, so that if the experiment be interrupted at the proper moment the other component, the /d/-leucine, alone is present and may be isolated in the pure state. In an actual experiment 3·8 grams of this component were obtained in the pure state from 10 grams of /dl/-leucine [Ehrlich, 1906, 1], so that the whole of the /l/-leucine (5 grams) had been decomposed but only 1·2 grams of the /d/-leucine. This mode of action has been found to be characteristic of the alcoholic fermentation of the amino-acids by yeast. In all the instances so far observed, both components of the inactive amino-acid are attacked, but usually the naturally occurring isomeride is the more rapidly decomposed, although in the case of β-aminobutyric acid both components disappear at the same rate [Ehrlich and Wendel, 1908, 1]. This reaction therefore must be classed along with the action of moulds on hydroxy-acids [McKenzie and Harden, 1903], and the action of lipase on inactive esters [Dakin, 1903, 1905], in which both isomerides are attacked but at unequal rates, and differs sharply from the action of yeast itself on sugars [Fischer and Thierfelder, 1894], and of emulsin, maltase, etc., which only act on one isomeride and leave the other entirely untouched [see Bayliss, 1914, pp. 55, 77, 117]. SUCCINIC ACID. The origin of the succinic acid formed in fermentation has also been traced by Ehrlich [1909] to the alcoholic fermentation of the amino-acids. It was shown by Buchner and by Kunz [1906] that succinic acid like fusel oil is not formed during fermentation by yeast-juice or zymin, and, in the light of Ehrlich's work on fusel oil, several [p090] modes of formation appeared possible for this substance [Ehrlich, 1906, 3]. The dibasic amino-acids might, for example, undergo simple reduction, the NH{2} group being removed as ammonia and replaced by hydrogen. Aspartic acid would thus pass into succinic acid:-- COOH·CH{2}·CH(NH{2})·COOH + 2H = COOH·CH{2}·CH{2}·COOH + NH{3}.