By intraperitoneal or intranasal inoculation of testical or cerebral emulsion of guinea-pigs infected with Rickettsia mooseri or R. prowazeki of mice and the cellular pictures of peritoneal exudate. The results were as follows: 1) By intraperitoneal inoculation, R. mooseri could produce manifest infection in all the seasons but that of high temperature, while R. prowazeki could not in any season. 2) By intranasal inoculation, both rickettsiae could cause the sickness in all the seasons except that of high temperature. R. mooseri, however, caused severer attack than R. prowazeki. 3) Even by intraperitoneal inoculation of R. mooseri of non-manifest infection, the serial passage of the rickettsia was well carried out. 4) After intraperitoneal inoculation of R. mooseri into mice in April, fibrocytes increased in the peritoneal exudate on the 1st to 4th day, fibrohistiocytes on th 5th to 7th day, and histiocytes on the 8th to 10th day. 5) In hunger-breeding mice previously injected with benzene and olive oil, intraperitoneal inoculation of R. prowazeki did not cause any manifest attack.