In the present study, an antibacterial action of chelating agents in use for the enlargement of root canals was examined. As a result of the diffusion test, Moruhonin, Largal and the agent by Ostby's prescription were found to possess antibacterial actions on the six bacterial strains. In addition, both 15.4% solution of the disodium salt of E. D. T. A. (pH 7.3) anc 0.76% Cetrimide (pH 7.3) which are main components of Ostby's preparation, possessed an antibacterial action on the same bacterial strains. The determination of the phenol coefficient by using Streptococcus mitis (St. 6-6) indicated that Ostby's preparation showed the largest antibacterial effect, order next being Largal and Moruhonin among these chelating agents. Between 0.76% Cetrimide and 15.4% solution of the disodium salt of E. D. T. A., the former showed a larger phenol coefficient than the latter.