The effects of a number of crystalline and highly purified enzymes on elementary bodies of vaccinia are reported. These effects have been followed by determination of amino nitrogen, staining reaction, and studies of infectivity.

Pepsin, at a pH which inactivates the virus, results in its solution and rapid release of amino nitrogen.

Crystalline trypsin, chymotrypsin, carboxypeptidase, and ribonuclease are without appreciable effect on the virus.

Papain within a short time produces profound alteration in the staining reaction of the elementary body with release of amino nitrogen accompanied by complete inactivation of the virus. This reaction is not shared by crystalline ficin, another plant papain, or by cathepsin, an intracellular proteinase analogous to plant papains but of animal origin.

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