With the purpose to study the degree of virulency of various drug resistant tubercle bacilli, the author injected true double and triple resistant tubercle bacilli obtained from tuberclous patients and also H(37)Rv double and triple resistant strain intraperitoneally into guinea pigs and compared their virulency. 1. On comparing the results of the group inoculated with double resistant bacilli with those in the group injected with susceptible tubercle bacilli, the virulency in the former group was markedly diminished. As for the difference in the virulency of double resistant bacilli according to the combination of drugs, a considerable fall in the virulency can be recognized in the combination of SM-PAS, but the decrease in the virulency is still greater in such combinations as SM-INH and PAS-INH that include INH. In other words, the virulency is controlled and represented by PAS resistance in the case of SM-PAS, whereas it is by INH resistance in the case of SM-INH and PAS-INH. In addition, in the combinations of SM-INH and PAS-INH about the some degree of the decrese in virulency can be recognized on the contrary to expectation, showing hardly any difference between the two. 2. There can be seen about the same degree of the fall in the virulency of the group inoculated with the double resistant bacilli obtained from patients and in that injected with H(37)Rv double resistent strain, showing on significant difference. 3. In the group inoculated with triple resistant bacilli the fall in the virulency is still greater than in the case with double resistant bacilli. Especially so in the triple resistant bacilli that is more resistant to INH. 4. There can be recognized no significant difference in the fall of virulency between the group inoculated with bacilli obtained from patients and the group injected with H(37)Rv triple resistant bacilli. 5. Really, the fall in the virulency of double or triple resistant bacilli is dependent on the drug resistance of the bacilli against INH or PAS, especially on the resistance against INH.