The antiviral effect of 9-methylstreptimidone (9-MS) was examined in mice infected with mouse-adapted influenza A2 (H2N2) virus. Both a single and continuous prophylactic administration of 9-MS protected mice from virus infection, and comparison between the minimal effective and the 50% lethal dose gave a therapeutic index of 60. When the treatment was started after infection, however, no antiviral effect was demonstrated. After a single intraperitoneal administration of 9-MS, a highly potent virus-inhibitory factor was detected in the lungs (10 h later) and the sera (16 h later) of uninfected mice, which was assumed to be an interferon on the basis of the biological characteristics. These results suggest that the protective activity of the antibiotic is due to interferon induction in mice.