While the use of chloramphenicol showed marked curative effect on tsutsugamushi disease, especially in the case of a large dose at the same time. WOODWARD and KATSURA expressed a doubt on this effect, above all KATSURA reported the possible occurence of re-infection of this disease following after the therapy by such method of dose. On the other hand, the repeated use of small dose of the drug might allow the pathogenic rickettsia to survive the action of the drug, also allow to cause the variation of the rickettsia. From the viewpoint of this, the author investigated the variation of pathogenicity of the tsutsugamushi disease rickettsia, OSEKI's and MITANI's strains, by action of chloramphenicol: the variation was determined by the use of LD(50) for a intraperaperitoneal inoculation of the organism on the previously treated mice, that were given chloramphenicol perorally for 10 days so as each mouse to have the different drose daily, namely 5 mg., 2 mg., 1 mg. and 0.5 mg. per 10 g. of body weight respedtively. Contrarily to the control, it was noticed the very low value for the LD50 of the test animals. Further the author conducted the study on the varied organism thus obtained, namely varied OSEKI's and varied MITANI's strains, in that the rickettsia was inoculated intraperitoneally to the guinea pigs that were previously injected 20 mg. of chlorpromazine per Kg. of body weight daily for 4 days; and the result of the test was determined on the basis of the amount of ascites produced, the amount of free cells and the presence of the rickettsia in the ascites. In this study, it was demonstrated that the infection of the original strain did not reveal the illness during summer and winter, but the varied strain could reveal the marked sign of illness at any time of seasons and the much production of ascites and marked proliferation of the rickettsia were observed in abdominal cavity. These facts showed supposedly that the marked variation was occured on the pathogenicity of the tsutsugamushi disease rickettria treated by chloramphenicol compared with the untreated organism.