In the preceding paper was reported a study of the influence of some extracts and filtrates of the pneumococcus and of a few chemical reagents on the virulence of pneumococci. It was shown that purpura developed in mice inoculated with the extract prepared by repeatedly freezing and thawing the pneumococci. If the mice received the extract twenty-four hours preceding the inoculation of a pneumococcus culture of borderline virulence, they were protected against the infection. This protection persisted only as long as purpura was observable. It was also reported that when fresh filtrates of virulent pneumococci were inoculated simultaneously with the culture of low virulence a large number of the mice at autopsy showed a fibrinopurulent pleuritis. The most potent filtrates were obtained when the cultures grew in the presence of salts that lower the electrophoretic potential of microorganisms. Complete autopsy records of the mice were kept and the tissues from a representative number were prepared for histological examination. The present paper comprises a study of the pathological lesions observed in these mice.Among the early reports on the pathological changes produced by the pneumococcus in mice is the report by Sprunt and Leutscher (1912). They studied about eighty mice and found, in those dying several days after injection of living and dead pneumococci, acute degeneration of the walls of some of the large blood vessels and local hemorrhage. They did not find this de-363 on July 14, 2020 by guest