Part I On the Production of Acetoin Using E. coli, A. aerogenes, Sal. typhi and Staph. aureus as test organisms, the auther studied on the production of acetoin by the growing cells and the resting cells of these microorganisms and the following results were obtained. 1) While E. coli did not synthesize acetoin on its growth, A. aerogenes showed marked production of acetoin on growth on the media containing glucose, lactate or pyruvate as C-source. In the case of Sal. typhi and Staph. aureus, a small production of that was observed. 2) An accumulation of pyruvate into culture media was rapidly decreased from about the resting phase in the culture of A. aerogenes that was capable of synthesize action. On the other hand, the accumulation of pyruvate was decreased its amount fairly gradually by E. coli that had no capacity of acetoin sythesis. 3) The resting cells of either species could produce acetoin to some degree at the expense of glucose, lactate, pyruvate and succinate. The optimum pH of this reaction was found to be at about 6.0. 4) From the study on the effect of pH on oxidation of pyruvate by the resting cells of E. coli and A. aerogenes, it could be postulated that the mechanism yielding acetoin from pyruvate in low pH could work more sufficiently at A. aerogenes, than at E. coli. 5) The production of acetoin was accelerated very highly by the simultaneous addition of glucose and pyruvate into the media, and also was accelerated by the addition of VB(1), Mg(++) or ATP. Part 2 On the Degradation of Acetoin Using the 4 strains of bacteria as in the preceding paper, part I, the author studied on the degradation of acetoin by the growing cells and the resting cells of these microorganisms. The following results were obtained. 1) All the microorganisms tested except Staph. aureus could grow by utilizing acetoin as C-source on the media containing the minimal nutritional requirement, namely N-source and vitamins. But Staph. aureus did not show the growth for a lack of capacity utilizing acetoin. 2) As peptone and acetoin was added simultaneously into the media, all the species of bacteria could degrade acetoin and yielded a large amount of pyruvate and a little amount of lactate as the metabolite. 3) Generally the resting cells of all species showed a fairly large O(2)-uptake at the expense gf acotoin as substrate. However, the O(2)-uptake was somewhat small in the reaction by Staph. aureus compared with by the other bacteria. 4) As for the oxidation products of acetoin by the resting cells of the microorganisms pyruvate was found to be a predominant metabolite. Acetoin had possibly been undergone oxidative decarboxylation in the first place and resulted in pyruvate.