The etiology of tetanus and gas gangrene presents a biological problem of an unusual nature. When the organisms which cause these diseases are deprived of toxin, by washing in tap-water saline or by washing and then heating, they are non-pathogenic to laboratory animals. In a recent investigation of the problem we have shown that soluble ionisable calcium salts enable detoxicated B. tetani, B. Welchii, B. oedematiens and vibrion septique to exert their pathogenic powers. We concluded that calcium owes this property to its local action on the tissues.