The present experiment has been conducted with a view to study the interaction of two species of bacteria from the metabolic aspects; and as a preliminary, O(2)-consumption and RQ have been measured: Bacteria used: E. coli communis, A. aerogenes, Staphylococcus albus, and Staphylococcus aureus. Substrates: glucose, pyruvate, acetate, and formate. Inhibitory agents: KCN, DNP and NaN(3). 1. Since the O(2)-consumption in the case of the substrate glucose, is accelerated most markedly, it signifies that there is an interaction. 2. The RQ in the case of the substrate glucose has shown a greater increase when two combinations of E. coli and Staph. albus and that of A. aerogenes and Staph. albus are made to act at the same time than in the cases of individual bacterium made to act separately; while the RQ in the combination of E. coli with Staph. aureus has rather tended to decrease, and moreover, its acetoin production has been found to proceed in parallel with the value of RQ. 3. In the case of the inhibitory agent KCN, a marked inhibition of the O(2)-consumption has been revealed, while in the cases of NaN3 and DNP, an acceleration. Especially in the case of NaN(3) an acceleration of O(2)-consumption has been found; and in the combination of E. coli and Staph. albus with A. aerogenes and Staph. albus, the O2-consumption has been accelerated and its interaction striking. 4. In the case of an addition of the inhibitory agent NaN(3), RQ has been found to give a rather similar result as that in article 2, though with a slight lowering tendency. 5. In the case without NaN3, the interaction in the acetoin production presents a similar tendency as that revealed in the case of RQ. With the addition of NaN(3), the acetoin production has been inhibited. Therefore, it seems that due to an inhibition of the pyruvate to acetoin pathway by NaN(3), no acetoin production can be observed while an accumulation of pyruvate is to be expected.