A stable, water-soluble substance which possesses potent antimycobacterial activity under certain conditions in vitro has been prepared from calf thymus. This substance has been tentatively named thymus peptide. In final concentrations of 1 to 10 µg. per ml. of an albumin medium it inhibits the growth of various strains of mammalian mycobacteria, but manifests only little or no inhibitory activity against a variety of other microbial species.

The ability of thymus peptide to inhibit the multiplication of tubercle bacilli diminishes when the inoculum is large, or when the medium is acidic. It is also markedly antagonized by addition of enzymatic hydrolysate of casein or beef heart infusion broth to the culture medium.

Thymus peptide does not exert a rapid bactericidal action on tubercle bacilli, but organisms exposed to this compound for longer than 2 weeks could not be made to multiply in ordinary culture media.

Substances similar or identical to the thymus peptide preparation could be extracted from calf spleen, sheep thymus, beef lymph nodes, and calf pancreas, but not from calf lung or calf liver.

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