A study is made of the relation of the chemical structure of the thiosemicarbazones to antituberculous activity. The S atom of the thiosemicarbazones plays an essential role. Thiosemicarbazones of aldehydes behave more favorably than do thiosemicarbazones of ketones. Substitution of the N atoms of the thiosemicarbazide grouping causes the action to disappear increasingly with the size of the substituents. The action can be increased materially, by the introduction of suitable substituents on the aromatic ring. Of the many corn-pounds studied, 4-acetylaminobenzaldehyde thiosemicarbazone has proved to be the most effective.