1. Mouse protective tests are described which permit the accurate and reproducible titration of the anti-infectious and antitoxic antibodies in hyperimmune antiplague rabbit sera. The potency of such sera can be expressed in provisional standard units based on a standard serum the use of which eliminates erroneous results due to fluctuations in mouse susceptibility, strain virulence, or environmental conditions.

  2. The sera of rabbits given 3 intravenous injections of live avirulent P. pestis on alternate days have a content of protective antibodies which is at a maximum within one week after the last antigen administration and falls to a more or less constant level 3 to 5 weeks later. Only exceptional animals can be pushed to a higher level by frequently repeated injections.

  3. No correlation is apparent between the antitoxic and protective titer of a given serum. It is certain, however, that a detectable level of antitoxic antibodies is attained in rabbits only if a re-stimulating injection is given 3 to 4 weeks after the primary series of injections of antigen. The re-stimulation induces a sudden rise in antitoxin-titer, but does not effect the level of protective antibody except for the production of a brief and fleeting negative phase.

  4. Mice immunized actively by the injection of live plague vaccine develop not only anti-infectious immunity, but also significant antitoxic resistance.

  5. The significance of the results and the possible role of serum antibodies in the mechanism of immunity to P. pestis infection are discussed.

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Aided by grants from the Rosenberg Foundation and the Columbia Foundation.

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