1. Only two serologically different proteins were found in the unheated culture filtrates of both virulent and slightly virulent tubercle bacilli. One of them was the protein which had a sedimentation constant of 3.4 S, and the other was in filtrate fractions with a constant of 2 S.2. That these proteins were distinct was demonstrated by three methods: quantitative precipitin and precipitin absorption tests with rabbit antisera, skin tests in guinea pigs actively sensitized with the culture filtrate fractions, and skin tests in passively sensitized guinea pigs.3. A third antigen of unknown nature was found by means of the precipitin tests, but only in certain fractions from the virulent culture filtrate.4. The protein with the constant of 3.4 S could not be demonstrated serologically in an O.T. made from the same culture filtrate as the unheated preparation from the virulent organism.
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