Concentrated culture filtrates of two strains of human tubercle bacilli, a virulent and a slightly virulent one, have been fractionated to give fourteen fractions in each case. Chemical determinations and sedimentation velocity measurements have been carried out on those fractions for which significant results could be obtained. The evidence is that two distinct proteins are present, in addition to a polysaccharide and nucleic acid. The physical measurements have not demonstrated the presence of any other proteins. One of the proteins has been isolated in pure form, and found to have a molecular weight of 44,000 ± 5,000, based on measurements of partial specific volume, sedimentation velocity, and diffusion rate. This protein is believed to be the same as one previously isolated by Seibert et al. (6), who assigned it a molecular weight of 32,000. The other protein was not obtained sufficiently free from polysaccharide so that its molecular weight could be determined, but it is believed to have a sedimentation constant of about 2 S. Sedimentation and diffusion constants have been obtained for the polysaccharide, which appears to be a homogeneous molecular species with a molecular weight of about 20,000. The source in unheated tuberculin of the proteins obtained from heated preparations is discussed.

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