1. Four general types of spontaneous pneumonia, associated with strains of Bact. lepisepticum similar in biology and virulence, are described: (1) an acute, diffuse form, with subpleural and perivascular orientation of the exudate, (2) lobar, (3) pleuro-, and (4) abscess pneumonia.
2. The acute, diffuse, lobar, and pleuro pneumonias may be induced experimentally by intranasal instillation of a virulent strain of Bact. lepisepticum.
3. These same types occur when the organisms are inoculated intravenously, intratesticularly, and subcutaneously.
4. Intrabronchial insufflation of the organisms brings about infection in less than half of the animals. When effective, a sharply circumscribed, peribronchial lesion is found at the base of the lung, which spreads peripherally by direct extension, and generally by invasion of the blood stream.
5. It is concluded that differences in the types of pneumonia following infection with similar strains of Bact. lepisepticum depend upon the resistance of the animal, and that the usual portal of entry of this organism into the lungs, in cases of acute and lobar pneumonia, is by way of the blood stream.