As there is no detailed report on the fundamental study concerning isonicotinic acid hydrazide (INAH) and two derivatives, isonicotinyl hydrazide metasulfonate (IHMS) and glucuronolacton isonicotinyl hydrazone (INHG), the author conducted in vitro studies with these three drugs and obtained the following results: 1. The action the three drugs, INAH, IHMS, and INHG at the concentration of 0.2 inhibits the growth of the standard strain of human tuberculous bacilli. Moreover, the antiseptic power of these drugs against tuberculous bacilli is stronger than that of SM, PAS or Tbl. 2. The resistance of tuberculous bacilli against there three drugs is acquired step by step quite easily and the rapidity of the acquiring of such resistance in the case of INHG seems to be slightly more rapid than the other two. 3. For preservation of the titer of the medium practically four weeks are about the limit, and when storing at the room temperature, one should exercise an extreme care. The disintegration of the medium due to warming even for a long duration of time occurs to a lesser extent as long as the temperature is not kept high. 4. As for the effect inhibiting the growth of tuberculous bacilli, combined use of INAH and SM is clearly discernible, and the effect of INAH combined with PAS can be observable only when PAS is used in the concentration of over 0.5 γ. The effect of combination of IHMS with SM or PAS and of INHG with SM or PAS is slightly inferior to that of INAH with SM-PAS.