In rabbits immunized by the injection of suspensions of heat-killed pneumococci the results obtained as regards not only the development of agglutinating and mouse protective antibodies, but also the persistence of these bodies in the blood depend to a considerable extent upon the route of immunization and the size of the inocula. Agglutinins may appear in the sera of all the rabbits except those inoculated subcutaneously, but in most instances they disappear within a short time.

Protective antibodies appear in the sera of all rabbits, no matter which route of injection be employed and they persist much longer than do the agglutinins. They persist longest when the injections are made intravenously or intraperitoneally and are of briefest duration when the injections are made subcutaneously.

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