Streptomycin-independent strains were isolated in vitro by a single step selection from a large cell population of a streptomycin-dependent strain of the tubercle bacillus designated as 18 b. These strains are not the same as the streptomycin-sensitive ancestor strain H2, but they can easily adapt themselves to higher concentrations of streptomycin. This adaptation is accelerated when Fe and pepton are added to growing environment or the oxygen pressure is lowered in the medium. Glycerol egg media are the far more favourable condition than Kirchner's agar slants for the adaptation. When caffein, 8-azaguanine or acriflavine is incorporated in sub-inhibitory concentrations into streptomycin-containing media, the adaptation is strongly or almost completely inhibited. Reversely, when those streptomycin-independent strains have once completed their adaptation to streptomycin, such purines inhibit their adaptation to streptomycinfree environment. In other words, the streptomycin-independent strains grown on streptomycin-containing media do not show any significant growth when subcultured onto the media free from streptomycin but added with the purines. The strains show a diauxie growth in response to added streptomycin when observation is made of the homogeneous growth in Tween-albumin medium. All these findings can be explained on an assumption that streptomycin-independent strains are neither streptomycin-sensitive norresistant, but they are reversely streptomycin-dependent. In response to the environmental conditions with or without streptomycin, those strains can make phenotypic adaptation expressing one of the two metabolic potentialities, nemely dependent or non-dependent to streptomycin. Caffein or 8-azaguanine can be considered to inhibit this phenotypic adaptation. The present observation are also discussed from the possibility that streptomycin-independent strains are a suppressor mutant.