1. Three subcutaneous injections of Pneumococcus Type II vaccine confer on monkeys a complete immunity against experimental Pneumococcus Type II pneumonia. A similar protection can be bestowed on monkeys against Pneumonococcus Type IV pneumonia by three subcutaneous injections of a vaccine prepared from the same strain of pneumococcus.

2. The subcutaneous injection of monkeys with three doses of Pneumococcus Type III vaccine confers a complete immunity against this type in only 50 per cent of cases (four out of eight monkeys vaccinated).

3. In spite of the immunity induced in monkeys by three subcutaneous injections of Pneumococcus Types II, III, and IV vaccine, specific protective bodies against the homologous types are not demonstrable in their serums when the vaccine is so administered.

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