As for the resistance to the infection of Rickettsia tsutsugamushi, the immunization with inactivated vaccine had been previously reported. In the present report, the methods to inactivate the vaccine, which are closely relating to its effect, were studied. Tanizawa strain of R. tsutsugamushi separated in the eastern part of Kagawa Prefecture was used as the test agent, and was inactivated with formalin, phenol and merzonine. The results were as follows: 1) The formol-and phenol-vaccine showed little toxic effect, and any symptomes or signs of the disease did not appear in the mice immunized with these vaccines. No accidental death due to the toxic effect of the drugs used for inactivation was observed. 2) The immunization with merzonine-vaccine showed a significant effect, but the toxic effect of merzonine also considerably appeared. 3) Both of formol-and phenol-vaccine showed a significant infectious protecting effect and did not cause any accidental death, but formolvaccine seemed to be more suitable for imminization. In short, of all the inactivated vaccines of R. tsutsugamushi, the formol-vaccine was the most excellent and the phenol-vaccine the next.