In order to investigate the details of the infectivity of R. tsutsugamushi, the brain, the lung and the spleen of rickettsia-infected mice were inoculated into healthy mice after preservation of the materials in various ways. Tanizawa strain of R. tsutsugamushi was used as the test agent. The results are summarized as follows: 1) The preservation temperature has a very great influence on the infectivity of R. tsutsugamushi; the higher the preservation temperature, the lower the virulence. 2) With the laps of preservation time, the virulence fallr. 3) The fall of the virulence of R. tsutsugamushi is different according to the sorts of organs of infected animals. 4) According to the conditions of organs, the fall of the virulence is different. 5) When preserved in refrigerated condition, the virulence of R. tsutsugamushi is the most stable and shows no noticeable decrease for about 90 days.