1. The normal lung of the mouse possesses the power of reducing markedly its content of Type I pneumococci within 3 hours after inhalation of the organisms in the form of fine droplets.
2. Lungs with fully developed influenza viral pneumonia not only fail to reduce the pulmonary content of pneumococci administered in this manner but, on the contrary, support their growth.
3. After intrabronchial inoculation into mice, influenza virus multiplies rapidly in the lung within 24 hours.
4. Criteria have been established for distinction between true viral lesions of the lung and changes due to the inoculation of diluents as vehicles for the virus.
5. 24 hours after inoculation of virus, there are no macroscopic lesions in the lung and the microscopic changes are due to the diluent.
6. Presence and multiplication of the virus in the lung 24 hours after inoculation have no apparent effect on the power of the lung to reduce rapidly its content of inhaled pneumococci.
7. The effect of the virus in lowering resistance to secondary bacterial infection appears to be due to the presence of the lesion produced by the virus.