Mice were infected by aerosol inhalation with influenza A/PR8/34 virus, and the kinetics of infection were monitored by the measurement of infectious virus, viral neuraminidase activity, and viral antigen as detected by enzyme immunoassay. Pulmonary levels of neuraminidase activity closely paralleled the infectious titers quantitated by standard egg inoculation techniques. Both viral neuraminidase activity and viral antigen increased in a dose-dependent manner during the early stages of the viral infection. After day 5, however, viral neuraminidase activity precipitously declined, whereas viral antigen levels remained elevated at high concentration for up to 60 days. Immunosuppressive treatment with cyclophosphamide resulted in the prolonged maintenance of peak virus titers without any additional increases in viral antigen. Previously infected mice were resistant to reinfection with homologous virus as evidenced by the lack of detectable viral neuraminidase activity and the lack of generation of additional viral antigen. These data define the temporal relationship between levels of infectious virus, neuraminidase activity, and viral antigen in an experimental model of influenza virus infection.