Eighty per cent of mice infected with a mouse-adapted strain of influenza virus died within 3 to 8 days after infection. Rimantadine given at maximum protective doses reduced mortality to 10%. This protection is dose related and can be demonstrated with doses from 4.5 to 24 mg per kg per day. Significant survival rates are shown by delaying treatment as long as 48 hr after infection. Lungs of treated mice have significantly less virus than those of controls at 24, 48, and 72 hr after infection. Antibody production as measured by hemagglutination inhibition is not different in treated and controlled mice. These results indicate that rimantadine is an effective prophylactic and therapeutic agent and that its activity is associated with decreased viral titers.