A method has been described by which a stable, serologically active substance has been isolated in a relatively pure state from tissues infected with vaccine virus. It has the characteristics of an alcohol-soluble protein which is not precipitated by boiling in a neutral aqueous solution. In a dilution of 1:640,000 it gives a precipitate when mixed with a serum containing antibodies against Craigie's S antigen of vaccine virus, but no visible reaction occurs when it is mixed with serum depleted of S antibodies by means of absorption.

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