Current hypotheses of the mode of bactericidal action of chlorine suppose a direct combination of some chemical 'species of the chlorine with the bacterial protoplasm producing a toxic organic complex (Porter, 1946). Although this view is a great advance over earlier concepts, it still lacks precision in terms of our present knowledge of the intermediary metabolism of cells. The marked efficiency of chlorine, exerting a bactericidal action in concentrations of 0.2 to 2.0 ppm in water, will at once classify it as a biologically active "trace substance." It may consequently be assumed to exert its effect upon the enzyme systems of the cell (Green, 1941). By investigation of the effect of chlorine on bacteria and on various enzymes, its bactericidal effect has been shown to depend upon the inhibition of certain essential enzyme systems, and the mechanism of this inhibition involves the powerful oxidative action of chlorine on the-SH groups of these enzymes. METHODS Active chlorine was estimated by the acid-iodine thiosulfate titration procedure (Calvert, 1943). Chlorine solutions, when not otherwise specified, were obtained by neutralizing and filtering a fresh solution of calcium hypochlorite. 'This investigation was conducted under contract with the Committee on Medical Research of the Office of Scientific Research and Development and at the request of the Office of the Quartermaster General.

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