By comparing the virulence of INAH-resistant bacilli obtained in one-step selection and that of the INAH-resistant bacilli stocked in our laboratory, and also by studying the virulence of the INAH-and SM-double-resistant bacilli and the relationship between the catalase activity and the virulence of INAH-resistant bacilli, the author obtained the following results. 1) In the experiment with guinea pigs both the INAH-resistant bacilli obtained by one-step selection and the stock INAH-resistant bacilli proved to not so virulent. 2) The virulence of double resistant bacilli grow weaker when SM-resistant bacilli acquired INAH-resistance, and that of INAH-resistant bacilli remain weakly virulent even when they acquired SM-resistance. 3) On examining catalase activity and the degree of INAH-resistance, on the whole bacilli become catalase-negative as their resistance increases, but some remain catalase positive even when their resistance is high. Therefore, the relationship between INAH-resistance and catalase activity is not absolute. 4) When catalase negative 10 γ-resistant strain is cultured successively in a modified medium, it turns catalase positive. Moreover, it has been proven that catalase negative strain turns to catalase positive due to peculiar conditions of the medium but not due to the decrease in resistance, because even when 10 γ/cc. of INAH is added to the modified medium, bacilli turn to catalase positive just the same. 5) In the study of the relationship between catalase activity and virulence to guinea pigs, it has been found that the virulence of catalase-negative l0 γ-resistant strain is weakened. Even in the case of l0 γ-resistant bacilli which are catalase positive (such as obtained from patient or isolated in a modified medium), their virulence is maintained. In the case of INAH-resistant strain if they are catalase positive, the virulence of the bacilli is maintained in a fair degree. Consequently, it is possible to regain the virulence artificially to a certain extent by changing catalase to positive in a modified medium. Furthermore, in the case of the bacilli isolated in a modified medium and possessing the same degree of catalase positive activity, the virulence proved to be higher when the resistance is decreased. This fact seems to indicate that the virulence of INAH-resistant bacilli is in a way correlated to the resistance irrespective of catalase. 6) The relationship between catalase activity and the virulence to mice can not be detected so clearly as in the case of guinea pigs.