A new factor essential for the growth of hemolytic streptococci of Lancefield's group A has been demonstrated. It has not been possible to identify this factor with any other known growth factor. The active substance occurred in liver as a water-soluble, alcohol-insoluble, non-dialyzable material. It could be rendered dialyzable by treatment in strong alcohol with HCl, but not by many less drastic procedures.

The most active concentrates prepared gave a maximal effect when approximately 10 gamma per cc. were added.

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