Above ten years have passed through since the chemotherapy of tuberculosis has begun, and mycobacteria resistant to three major anti tuberculous drugs, i.e., streptomycin, isoniazid and paraaminosalicylate (PAS), are now of very common occurrence. It appears very reasonable that viomycin(1-9) has increased its importance in the chemotherapy of tuberculosis. Nevertheless, only a little knowledge has been obtained on the tuberculostatic action of viomycin. The mode of emergence of viomycin resistance in mycobacteria3,4,6), the absence of the cross resistance between viomycin and streptomycin or PAS2,6,7), the in vitro combined effect of viomycin with streptomycin or PAS10) have been reported by several authors, and these results suggest that the combined chemotherapy of viomycin with streptomycin or PAS retards the emergence of viomycin resistance. However, it should be remarked that, in the present state, viomycin is utilized ordinarily after the administration of the three major antituberculous drugs and that the resistance to one, two or three of the major drugs occurs very commonly, when viomycin must be administrated. Therefore, it appears much more important to study the mode of emergence of viomycin resistance in mycobacteria resistant to the major drugs and to study whether the tuberculostatic action of viomycin is favorably affected by the combined use of the major drugs, to which the organisms are resistant. It is the purpose of the present paper to concern with these problems from bacteriological aspect.