1. In vitro phagocytic experiments with human blood, antipneumococcus serum, pneumococcus specific soluble substance, and living virulent pneumococci show that there is a definite phagocytic inhibition zone when strong antiserum is used.

2. If the antiserum is further diluted, there is a zone where phagocytosis is effective. If the serum is diluted still more, phagocytosis gradually falls off, as the very dilute antiserum fails to neutralize the specific carbohydrate, which has a specific antiphagocytic action.

3. The inhibition zone is apparently caused by the specific precipitate (formed by the antiserum and the specific carbohydrate) interfering, perhaps mechanically, with the ingestion of the pneumococci by the leucocytes.

4. The inhibition zone is better marked with Type III than with Type I pneumococcus.

5. As the concentration of antiserum in the zone of effective phagocytosis in vitro does not correspond with the concentration of antiserum generally used in vivo in the serum therapy of pneumonia, this question is discussed.

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