By isolating in vitro tubercle bacilli of H(37)Rv strain each resistant to SM, INH or PAS, composed of an equal population, and by inoculating these resistant bacilli into guina pigs hypodermically, the author conducted a series of experiments on the virulency of these organisms, and at the same time pursued clinically and statistically the changes in resistance of bacilli after acquiring the resistance as well as the clinical progress. The results are described in the following. 1. The virulency of SM-resistant bacilli is found to be about the same as that of susceptible bacilli, showing entirely no decrease in the virulency. Furthermore, there exists no fixed relationship between the degree of resistance and the virulency, but the virulency is quite high irrespective of the degree of resistance. 2. The virulency of INH-resistant bacilli is markedly decreased; moreover, it is lower in the 10 γ/cc resistant bacilli than in the 1 γ/cc resistent one, proving to be almost powerless, and it is revelaed that the higher the resistance conversely the lower is the virulency 3. The change of virulency in PAS-resistant bacilli is not so marked as that in INH-resistant bacilli, but it is clearly down nervertheless. However, no fixed correlation can be recognized between the resistance and virulency. 4. The catalase activity is either decreased or obliterated only by INH-resistant bacilli, and therc can be recognized a certain definite correlation between the activity and the degree of resistance as well as the virulency. 5. In pursuing the changes of resistance clinically and statistically, it is fixed and unchanged in the majority of SM-resistant bacilli; it is changeable and lowered in most of INH-resistant ones; and it is between the two in the case of PAS-resistant ones, showing a fall in the resistance. This seems to be due to the proliferative power of resistant bacilli in vivo, indicating a strong proliferative power in SM-resistant bacilli while a weakened proliferative power in INH-and PAS-resistant bacilli, which fact coincides well with the results of animal experiments. 6. As for the clinical course after acquiring the resistance, in general the INH-resistant cases and the PAS-resistant show a satisfactory progress, which agrees well with the results obtained in animal experiments, suggesting that even in the human body the virulency of bacilli is lowered to a certain degree.