1. Rabbits recovering from one attack of experimental pneumonia possess an active immunity. Such animals may subsequently withstand repeated increasing doses of pneumococci intratracheally.
2. Death may supervene after any one of subsequent injections, but it seems to depend partly upon the chronic changes in the cardiorespiratory apparatus. It may at least be said that it is usually unassociated with a septicemia which is an invariable accompaniment of fatal primary lobar pneumonia.
3. The serum from animals actively immunized by the repeated intratracheal inoculations with pneumococci may be used successfully to confer a passive immunity against the homologous organism.