Isonicotinic acid hydrazide derivatives and allied compounds were prepared and the antibacterial tests in vitro were carried out with Mycobacterium tuberculosis A. T. C. C. No. 607. In general, it may be said that the hydrazone-type compounds of INAH showed antibacterial power about equal to that of INAH, methyl furyl-2 ketone isonicotinylhydrazone showing especially strong antibacterial action, with effective inhibitory concentration of 10^-8mole. Such a fact suggests that the presence of an isonicotinyl group in the molecule is necessary in order that these compounds show antibacterial action. The hydrazide of p-aminosalicylic acid also showed some antibacterial action though the action entirely disappeared on substituting the hydroxyl with chlorine, methoxyl, or ethoxyl. The antibacterial action appears again if the amino group is changed to the nitro group.