A quantitative biological assay of the products obtained from the dialysis of B. typhosus, meningococcus, and B. coli culture filtrates has been undertaken. It was found that the active principles of the phenomenon of local skin reactivity to bacterial filtrates were retained by cellophane membranes. An appreciable purification was thus effected, amounting, on the average, to about a threefold increase in reacting potency per milligram of dry weight and of nitrogen.

Attempts to purify bacterial filtrates by the fractional precipitation of their concentrates with dilute alkali, acid, and alcohol were unsuccessful.

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