Both of aureomycin and terramycin belong to the tetracycline, and their action has been considered to be nearly the same. The authors made a comparative study on the action of aureomycin and terramycin and obtained the following results: 1) The respiration of Escherichia coli in the presence of glucose, pyruvate, aspartate and glutamate is markedly inhibited by both of aureomycin and terramycin. The respiration inhibitory action of aureomycin is quantitatively about 3 times higher than that of terramycin. 2) The respiration of Micrococcus pyogenes var. aureus (Terashima) in the presence of glutamate, alanine, pyruvate and lactate is inhibited by both of aureomycin and terramycin. As for the respiration inhibitory action, aureomycin is quantitatively about 3 times stronger than terramycin. 3) The respiration of M. pyogenes var. sureus (Terashima) in the presence of glucose is not inhibited by aureomycin or terramycin, but is rather accelerated by these, particularly by terramycin. 4) As for the growth inhibitory action, aureomycin is somewnat stronger than terramycin.