The bacterial enzyme which decomposes the purified capsular polysaccharide of Type III Pneumococcus in vitro also destroys the capsules of the living organisms growing in media and in the animal body.

Potent preparations of this same enzyme protect mice against infection with virulent Type III Pneumococcus. The protective action is type-specific.

The protective activity of the specific enzyme is destroyed by heat (70°C. for 10 minutes).

The enzyme remains in an effective concentration 24 to 48 hours after its injection into normal mice.

The enzyme has been found to exert a favorable influence on the outcome of an infection already established at the time of treatment.

A definite relationship has been found to exist between the activity of the enzyme in vitro and its protective power in the animal body.

The mechanism of the protective action is discussed with special reference to the relation between the decapsulation of the bacteria by the enzyme and the phagocytic response of the host.

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