The specifcity and the nature of the phenomenon of local skin reactivity to various microorganisms have been studied. It has been shown that the skin preparatory and reacting factors of various biologically and serologically unrelated microorganisms are able to substitute for each other, provided they have the power of eliciting the phenomenon for themselves.

Additional evidence has been brought concerning the antigenic specificity of the factors eliciting the phenomenon. A variety of nonbacterial substances which are able to increase the permeability of capillaries, elicit inflammation and "block" the reticulo-endothelial cells; failed to induce the state of local skin reactivity to B. typhosus culture filtrate. Non-bacterial protein substances (crystalline egg albumin and normal horse serum) failed to reproduce the phenomenon.

It was not possible to obtain passive transfer of the local skin reactivity.

Various conditions influencing the potency of the bacterial culture filtrates have been pointed out.

The essential nature of the phenomenon has been discussed together with its significance in relationship to disease.

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