The power of specific antipneumococcus serum to protect mice against infection with Type I Pneumococcus has been studied with reference to the capacity of the animal to utilize the specific antibodies. With a single strain of mice it was found that smaller animals and those with large numbers of white cells in the peritoneal cavity are much better able to utilize the passively conferred immune principles.

These two intrinsic factors were resolved into a single element; namely, the number of monocytes in the peritoneal cavity at the time of injection of culture and serum. The interrelation of the extrinsic and intrinsic factors participating in the process of protection have been discussed.

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