In carrying out test inoculations in guinea-pigs with B. diphtheriae and diphtheroid bacilli for purposes of identification, where control animals were injected with the particular strain plus a certain amount of diphtheria antitoxin, it was noted that these animals were protected against lethal doses of B. diphtheriae by normal horse serum as well as by the specific immune serum. Though this observation did not involve any quantitative comparisons between the effect of the normal and the immune serum, the fact that normal horse serum should act protectively in the same way as the serum of a specifically immunized animal seemed of considerable importance and suggested the further investigation of the phenomenon. This normal serum effect is also of general interest in relation to “non-specific therapy” where alien protein (e.g., animal serum, bacterial protein, etc.) injected parenterally is found to exert a favourable influence on the course of various infections in a non-specific manner.

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