Abstract
Summary The observation of Pappenheimer and Johnson that abundant formation of diphtherial toxin occurs only in a relatively narrow zone of concentration of iron has been thoroughly verified. It has not proved possible to shift or extend this zone by the addition of other inorganic or organic materials (except manganese salts which seem partially to take the place of iron). On the other hand, addition of relatively large quantities of iron does not completely prevent toxin-formation, but rather reduces it to a comparatively low level. It is suggested that such a level represents normal production, whereas the larger amount obtained under conditions of iron starvation is the result of a compensatory mechanism in which the iron-free toxin molecule takes part in some process which is ordinarily catalyzed by an iron-containing enzyme.
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