With a view to elucidate the mechanism of reversibility of resistant tubercle bacilli, the author has performed in vitro experiments of resistance reversibility by observing changes occurring in successive culture of resistant bacilli together with different antituberculous agents, the filtrate of sensitive strain or dead bacilli, by changing ingredients of successive media; and also by injecting different kinds of antituberculous agents into imice the rever sibility in vivo has been studied. As the result the following interesting data were obtained. 1) In the successive cultures of SM-resistant H(37)Rv-strain and INAH-resistant-H(37)Rv-strain in the medium of Kirchner's albumin or serum liquid medium, each loaded with PAS, Viomycin, Pyrazinamide, INAH or SM, no reversibility of resistance could be recognized. 2) In the successive cultures of SM-resistant H(37)Rv-strain and SM-resistant avian type Takeo strain in the medium loaded with the filtrate of sensitive parent strain or dead bacilli, no reversibility of resistance could be observed. 3) In the cases of successive cultures of SM-resistant-H(37)Rv-strain and INAH-resistant-H(37)Rv-strain in modified media of 1% Ogawa's medium, SM-resistant bacilli revealed no resistance-reversibility in any of these media. INAH-resistant strain successively cultured in the medium containing solely KH(2)PO(4) or only sodium glutamate, they began to show a diminution in resistance about the fifth generation. As for the mechanism of this diminution in resistance it is possible to assume an induced backmutation in modified media. 4) In the experiments with mice, the diminution in resistance has been observed in the cases inoculated with SM-resistant strain and injected with INAH as well as in the cases inoculated with INAH resistant strain and injected with SM. It is assumed that the susceptibility of resistant bacilli to other agents is elevated in vivo.