In this paper there are described methods for grading the local skin reactivity induced by bacterial filtrates. The grading is accomplished by titration of the skin-preparatory factors; and by studies on the duration of ensuing reactivity.

It was found that the duration of reactivity depends upon the mode of preparation of filtrates and the microorganisms employed. Thus, it lasts for 96 hours with meningococcus "agar washings" filtrates; for 72 hours with B. typhosus "agar washings" filtrates; it disappears within 48 hours with B. typhosus tryptic digest broth culture filtrate and B. typhosus "agar washings" filtrates previously heated in the Arnold sterilizer for 20 minutes.

Comparative titrations of the preparations employed demonstrate that the duration of reactivity is in direct relationship to the reacting potency.

It is also shown that the skin-preparatory potency of the filtrates and the duration of the ensuing local reactivity are not modified by cellular anaphylactic sensitization (Arthus phenomenon) to animal proteins.

The exposure of tissues to the effect of certain soluble bacterial factors induces a high susceptibility to humoral toxic principles resulting from intravascular antigen-antibody interaction.

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