IT has been shown1 that in most bacterial cells acetoin is formed by the condensation of pyruvate and activated acetaldehyde. The findings of Beck-horn2, De Man3 and Mizuno and Jezeski4 leave no doubt that in the heterofermentative genus Leuconostoc the synthesis occurs via acetolactate. It is still uncertain, however, if acetoin is formed from sugar and, if so, out of which part of the carbon chain of the sugar. Preliminary experiments showed that washed cell suspensions of Leuconostoc citrovorum ATCC 8082 do not form acetoin during glucose fermentation in a phosphate buffer. On the other hand, a large amount of acetoin is produced from pyruvate. When pyruvate and glucose are added simultaneously, the synthesis of acetoin is inhibited. Only at the end of the fermentation, when the glucose concentration becomes so low that the production of carbon dioxide decreases, does the synthesis of acetoin begin. In the presence of a favourable H-donator (glucose) the pyruvate in the cell is presumably reduced so quickly to lactic acid that the condensation of pyruvate to acetolactate does not occur.