The mechanism of bactericidal action of hypochlorite and chloramine has been a very important and interesting problem to be investigated, but it has been still in an early stage of study. As the development of the investigation of the mechanisms of cytotoxic effect mediated by phagocytosis of neutrophil leukocyte, participation of hypochlorite to the cytotoxic effect and its production by myeloperoxidase in neutrophil granules have got into the spotlight. In the present paper, the mechanisms and roles of biological production of hypochlorite in mammalian cells and tissues are reviewed, and authors want to make clear the mechanisms of hypochlorite action in cytotoxicity and in disinfection.