In order to find out the actual percentage of the triple resistant clones of the so-called three-drug resistant strains, isolated from 15 patients, the author conducted a series of culture of these bacilli by the combined medium method and also by the cross culture method of a single clone, and obtained the following results. 1. The percentage of the three-drug resistant clone in the three drug medium proved to be 4.2 per cent in the medium composed of 10γ SM-1γ PAS-1γ INH; 7.2 per cent in the medium of 100γ SM-10γ PAS-1γ INH; and 2.0 per cent in the medium of 100γ SM-100γ PAS-10γ INH, demonstrating the percentage lower than in the case of the double resistant strain as described in the previous report. 2. In the case with the cross culture of a single clone, the percentage of the triple resistant bacilli is 10.0 per cent in the medium of 10γ SM-1γ PAS-1γ INH; 6.0 per cent in the medium of 100γ SM-10γ PAS-1γ INH; and 2.5 per cent in the medium of 100γ SM-100γ PAS-10γ INH, showing the values close to those in the combined medium culture. 3. The percentage of each triple resistant strain is a little less than the product of the percentage of three individual clones. 4. It is difficult to carry on the successive transplantation of any one of these triple resistant strains, and this seems to be due to a marked decrease in their proliferating power.