In order to make clear the mechanism of how the INH resistance weakens under the influence of sulfa drugs, the author conducted experiments by using the method of mixed population and also tested the susceptibility of INH resistant bacilli and INH receptive bacilli to sulfa drugs and their gaining of resistance against sulfa drgus. By such experiments, the author obtained the following results. 1. In the case of studying the clone of a perfect INH resistant strain for generations cultured in a SI-added culture medium, the author observed no weakening of the resistance. 2. Rapid weakening of INH resistance was found in the bacilli cultured in a SI-added medium for generations by use of the method of mixed population. Assuming that this phenomenon of weakening might be attributable to the difference between the susceptibility of INH resistant bacilli to SI and that of INH receptive bacilli to SI, the author further examined but was unable to find any bacterioseptic action in either bacilli by adding 500γ of SI. As regards bacteriostatic action, no difference was observed in the case of small inoculation of bacilli. In the case of massive inoculation of bacilli difference of some measure appeared. The results of the experiments seem to indicate that, although there is no difference in the susceptibility itself between INH resistant bacilli and INH receptive bacilli to SI, SI had delicate effects in delaying the growth of INH resistant bacilli. 3. The author studied the bacteriostatic action of long acting sulfa drugs such SD, SIM, SP and SMP and found that the action of SD and SIM was most violent, about twice as much as that of SI. The bacteriostatic action of SP was a little milder than that of SD and SIM and that of SMP most mild. As regards the bacteriostatic action of long acting sulfa drugs such as the ones quoted in the above on INH resistant bacilli and INH receptive bacilli, no significant difference could be found. 4. No difference was observed in the process of gaining resistance against SI between INH resistant bacilli and receptive bacilli. The resistance of neither bacilli increased beyond 125γ.