The detailed report on viomycin (VM), the second anti-tuberculous antibiotics next to SM, is rare, and none at all on its use in combination with INAH or with INHG. Therefore, the author performed the in vitro experiments, acute toxicity tests, and VM treatment against mouse tuberculosis, and obtained the following results. 1. VM solution at the concentration of 15.0 inhibits the growth of tuberculous bacilli, and it acts against SM-resistant strain identically as against the SM-susceptible strain. 2. However, VM used in combination with SM, INAH, or INHG, acts as to assist one another, while VM combined with PAS has no effect at all. 3. LD(50) of VM in the case of subcutaneous injection to mouse is 1,626 mg/kg (±157.64 mg/kg). 4. For the mouse berculosis the effect of VM alone is weak, while VM and SM nsed in equal amount separately the effect of VM is about one half that of SM alone. 5. The use of VM comb ned with INAH or INHG is super ro to the effect of VM used alone, showing no significant difference from that of INAH or INHG used alone thus proving the addition of VM is ineffective. From these it is assumed that VM is effective to tuberculosis in the case only when the causative strain has the resistance against SM or INAH.