Whenan animal has received repeated injections of foreign protein it reacts with acute inflammation often followed by necrosis whenthe antigen is reinjected into the skin. This reaction, usually designated the Arthus phenomenon, has been described asskin hypersensitiveness, local anaphylaxis, etc. It is not limited to the skin,for tissues of internalorgans,for example, the lungs or serous membranes, exhibit a similar heightened inflammatory reaction in consequence of repeated protein injections. The reaction makes its appearancewithin ten daysafter the injection of protein but doesnot reach maximum intensity until the animal has received six or eight injections at intervals of several days.No relation to anaphylactic shock has been established. The Arthus phenomenon resembles very closely the tuberculin, luetin and typhoidin reactions which are produced when poducts derived from a microörganism are injected into an individual who has been invaded by the corresponding microörganism.