This experiment was performed to investigate the effect of aureomycin, chloromycetin, and streptomycin upon the oxidation of several substrates by Sh. flexneri 2a and E. coli communis. The results are as follows: 1) A level of 200γ/ml aureomycin markedly inhibits the oxygen uptake with substrates such as glucose, pyruvate, glutamate, and aspartate. The inhibitory rate has no relation to the preincubation time of a mixture of aureomycin and a cell suspension. 2) A high concentration (200γ/ml) of chloromycetin slightly inhibits the oxidation of amino acids such as glutamate and aspartate. The inhibitory rate has a relation to the preincubation time of a mixture of chloromycetin and whole cell suspensions. 3) A level of 200γ/ml streptomycin slightly inhibits the oxidation of glutamate and aspartate by E. coli, but it does not affect the oxidation of glucose and pyruvate by this organism. The oxidation of proposed substrates by Sh. flexneri 2a is not also prevented by streptomycin. Even though streptomycin is added to the original cell suspension before the substrates and preincubated for an hour, the results are the same.