A new antiviral agent Virazole (1-B-D-Ribofuranosyl-1, 2, 4-triazole-3-carboxamide) was tested for its antiviral effect against influenza A virus infections both in vitro and in vivo. Withoutt any virocidal effect, virostatic effect against PR 8 strain was evident in HeLa S₃ cells at 25 meg/ml, the concentration which shows a definite but reversible inhibitory effect on the multiplication of host HeLa cells. In vivo effect against the same influenza A infections in mice was evident at the dose of 25mg/kg when given intraperitoneally 3 times before infection, 4 times on the day of infection and twice daily for consecutive 4 days after infection. A total of 50mg/kg dose under the same schedule revealed toxic signs to mice.