Immunization with inactivated vaccines of Rickettsia tsutsugamushi has not been succeeded as yet, and also no definite result has not been reported on its inactivated vaccines. The author studied the prophylactic effect of crude vaccine of R. tsutsugamushi. The test agents were Tanizawa and Mitani strains of R. tsutsugamushi separated and identified in the eastern part of Kagawa Prefecture. The formol-vaccine of the two strains was made of the peritoneal exudates containing a great deal of the agents which were obtained by intraperitoneal inoculation of rickettsiae into guinea-pigs previously injected with chlorpromazine. Mice were used as the test animals. The results were as follows: 1) The vaccine tested did not show any toxic effect against mice, and the vaccinated mice did not show any symptomes or signs of the disease or accidental death. 2) The mice thus vaccinated showed apparently the resistance to the attack of the homologous strain. 3) Against the heterologous strain, the resistance to infection was observed to some extent, but was far inferior compared with that against the homologous one. In short. the infectious protection by the formol-vaccine was highly strain-specific, and showed a high resistance against the attack of the homologous strain.