The effects of graded doses of ribavirin administered either by aerosol or intraperitoneally were compared in influenza virus-infected mice. The median effective dose values (based upon percent survival) were 3.3 and 15.8 mg/kg per day for the aerosol and intraperitoneal routes, respectively. Lung lesion scores and titer of virus were lower after aerosol than intraperitoneal therapy.