By performing in vitro and in vivo experiments with pyrazinamide (Pza) separately and in combination with other drugs, the author obtained the following results: 1. The concentration at which Pza, acting alone, inhibits the growth of H(37)Rv strain proves to be 200 γ/1cc. 2. Pza and SM in combination act quite collaboratingly, but either one of these in combination with PAS has no effect inhibiting the growth of tuberculous bacilli at all. However, Pza in combination with INAH or INHG presents a multiple action assisting one another. 3. The therapeutic effect to experimental mouse tuberculosis of Pza (250 mg/kg) alone is inferior to that of INAH (50 mg/kg) or INHG (500 mg/kg) given separately. 4. The effect of the treatment with Pza (250 mg/kg) + INAH (50 mg/kg), or Pza (250 mg/kg) + INHG (500 mg/kg) is superior to that of INAH or INHG alone, but this effect is not so efficacious enough to be valued as a bacteriocidal treatment in vivo. Moreover, the combination of Pza with INHG is slightly inferior to the combination of Pza with INAH. 5. The use pf Pza (250 mg/kg) alone on the mouse tuberculosis induced by the infection of INAH-10-resistant strain gives hardly any therapeutic effect. 6. The combinations of Pza-INAH and Pza-INHG are effective for the treatment of mouse tuverculosis induced by the infection of INAH-10γ-resistant strain. The effect of the combination therapy is slightly better than that of INAH or INHG alone, but the combination therapy of Pza with INAH does not differ from that of Pza with INHG in effectiveness.