1. The mechanism of sensitization of the isolated frog' heart to a product of the hemolytic streptococcus has been investigated. The sensitization depends upon the release from the isolated heart of a substance which inhibits the action of the cardiotoxin. The inhibitor is released upon the first (sensitizing) administration of cardiotoxin, but not upon the second (contracture-producing) administration.

2. The inhibitor released by the heart neutralizes the lethal factor present in the streptococcal preparation employed.

3. Some of the properties of the inhibitor have been investigated.

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