Cells and cell-free extracts are able to dissimilate pyruvate, anaerobically, by two different mechanisms. The predominating mechanism depends upon the method in which the cells were grown. When grown In a medium containing no glucose, the cells, and hence the cell-free extracts, dissimilate pyruvate primarily by the dismutation reaction. Cells grown in the presence of glucose dissimilated pyruvate to form carbon dioxide, acetylmethylcarbinol and 2,3-butyleneglycol in addition to the products of the dismutation reaction. Since the reaction by which acetylmethylcarbinol is formed Is much faster than the dismutation reaction, a large proportion (75 percent) of the pyruvate is dissimilated by this reaction;The enzyme producing aceetylmethylcarbinol from pyruvate requires diphosphothiamine but no phosphate. The enzyme system concerned with the dismutation reaction seems quite insensitive to diphosphothiame;Acetyl phosphate is not formed In the dismutation reaction;The dismutation reaction is reversible with respect to lactate and carbon dioxide. Acetate does not take part in the reverse reaction. Adenosine triphosphate enhances the fixation of carbon dioxide, whereas no effect was found with acetate;Phenylpyruvate acts as a competitive inhibitor of the anaerobic dissimilation of pyruvate by A. aerogenes. (Acetyl-methylcarbinol and carbon dioxide are produced by this reaction.). Phenylpyruvate inhibits, specifically, the anaerobic metabolism of pyruvate, the greatest inhibition being observed with those preparations which produce acetylmethylcarbinol and carbon dioxide from pyruvate;The constant for the dissociation of the enzyme-substrate complex In the acetylmethylcarbinol reaction is approximately 5.0 x 10-2 . The constant for the dissociation of the enzyme inhibitor complex is of the order of 4.0 x 10-5. The reciprocals of the dissociation constants, the affinity constants, show that the enzyme combines much more firmly with the inhibitor than it does with the substrate;The fact that the substrate concentration must be squared in calculating the dissociation constant indicates that the enzyme reaction involves two molecules of substrate rather than just one. In A. aerogenes this fact precludes the Neuberg scheme for the formation of acetylmethylcarbinol.