1. A study was made of the behavior of the blood platelets during the course of pneumonia in man, as also in several rabbits experimentally infected with pneumococci.

2. The number of platelets begins to diminish after the infection has become established. A thrombopenia occurs during the febrile period. Immediately following the onset of the crisis in lobar pneumonia or after the temperature begins to fall by lysis, the platelets begin gradually to increase in number. In the postfebrile period, the platelet count increases until the normal number is greatly exceeded, and there is a return to the normal number only after about 2 weeks.

3. Sequelæ, characterized by fever, following pneumonia are accompanied by a renewed diminution in the number of platelets. After subsidence of the temperature their number usually increases until the normal is again exceeded.

4. The platelets in general are smaller and contain more granules during the thrombopenic stage of pneumonia.

5. Intravenous injection of antipneumococcus serum during pneumonia ordinarily produces no variation in the number of platelets.

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