In order to see the relation between the catalase activity and virulence of INH resistant bacilli, the author studied the clone of INH resistant bacilli in vitro, looked into its catalase activity and the change of activity as generation descends and tested the virulence by inoculating guinea pigs with each generation of the bacilli. The results found were as follows: 1. The INH resistance bacilli obtained in one step was found to have a resistance potential of 0.05γ or more and to have lost its catalase activity. At this stage the selectivity of INH resistant bacilli which has a resistance of 0.05γ-10γ and the population composition of each group were similar. Therefore it is believed that the metabolic system of INH resistant bacilli made available by means of selection by use of drugs is different from that of receptive bacilli, and that the loss of catalase activity is attributable to this difference. 2. It was found that the negative catalase activity of INH 10γ resistant strains using the clone can be changed to positive by changing the conditions of the culture medium. Other multiple resistant strains which have the INH resistance are all C (-) and could not be changed to C (+) by changing the conditions of the culture medium. Therefore it is presumed that there is greater difference in the metabolic system between a single resistance strain which resists only INH and multiple resistance strain which resists not only INH. 3. A H(2)O(2) resistance test was given in respect of C (-) INH resistant bacilli and INH resistant bacilli the catalase activity of which had been changed from negative to positive and found that the resistance of both bacilli weakens compared to that of receptive bacilli and also that even the catalase activity of INH resistant bacilli changed from negative to positive is far weaker than that of receptive bacilli. 4. The author tested the virulence of C (-) INH resistant bacilli and INH resistant bacilli the catalase of which changed from negative to positive on guinea pigs, and found that the virulence of both had weakened. The author also tested virulence of each generation of cultured bacilli and found that it had lost its virulence. Juding from the results of the tests, it appears that the weakening of the virulence of INH resistant bacilli is an inherent property of the bacilli itself. However, the results of the H(2)O(2) resistance tests seem to indicate that the relation between the virulence and catalase cannot be completely denied. Further the author inoculated guinea pigs with C (-) INH resistant bacilli and some with C (+) INH resistant bacilli and separated them into two groups, one group killed at 3rd week and the other group killed at 8th week after the inoculation and found many cases of local disorder in the group killed at 3rd week irrespective of the kind of catalase, which led to believe that the disorder of guinea pigs attributable to INH resistant bacilli is reversible.