1. The intratracheal injection of egg albumin or pneumococcus protein induces an inflammatory reaction in the lungs of rabbits previously inoculated with the respective antigen.
2. A similar reaction occurs following intratracheal injection of pneumococcus protein into the lungs of rabbits previously inoculated with heat-killed suspensions of the bacteria.
3. This reaction appears to be related to the presence of circulating antibody and to have the nature of the Arthus reaction.
4. A study of the reaction of the lung of rabbits to infection caused by intravenous injections of Pneumococcus reveals that (a) reactions occur irregularly in the lung; (b) in the lungs in which reactions do occur, the histological changes are not different in normal rabbits and in rabbits made resistant by previous intravenous or intracutaneous injections of pneumococci.
5. Intratracheal injection of pneumococcus protein followed by intravenous injection of virulent pneumococci on the next day does not alter the course and character of the infection in resistant rabbits.
6. The experiments reported in this paper bring no evidences to support the view that the lesions in the lungs of rabbits following the intravenous injection of pneumococci are modified by any previous state of sensitivity.