The mouse protection test for the evaluation of type specific antipneumococcus serum has been studied with reference to the quantitative relationships between the amount of antibody and the number of injected bacteria. By varying both these factors through wide limits certain definite characteristics of the protection test have been defined. These are the Schwellenwert, the prozone, and the limiting titer zone. The modifications produced in the manifestations of these characteristics by changes in the technic of performing the test and also by certain non-specific reagents are described. The dependence of the outcome of the test upon variable factors intrinsic to the host is discussed.

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