In this report the proliferation of the newly isolated R. tsutsugamushi strains in the yolk sack was studied. Moreover, using the well-proliferating strain, the immunological natures were also investigated. The results are as follows: 1. Proliferation and fixation in embryonated egg yolk sack are varied according to the sorts of strains: namely two strains from rats gradually showed good naturalization and fixaiont by successive inoculation, but those from patients could not show any prolifiration 2. In infection-protective test, using the rickettsiae prolifirated in yolk sack and the antigens prepared from the infected animal organs, the immune effect of the former seemed to be inferior to that of the latter. This will be due to the difference of proliferation of rickettia. 3. In order to prepare the antigen of complement-fixation test, varioùs methods were tried, of which the method with organic solvents seemed to be relatively simple and reliable.