The ascites of the guinea-pigs previously injected with chlorpromazine and then inoculated with Rickettsia tsutsugamushi was separated into two fractions, tissue and rickettsial fractions, from which formol vaccines were prepared. Each of these formol vaccines was subcutaneously injected into mice, and the relationship between infectious protecting power and complement-fixing antigenicity was investigated. In regard to the immunity, the vaccine of tissue fraction was not inferior to that of rickettsial fraction, and the complement-fixing antigenicity of each revealed no marked difference. In either vaccine of these two fractions, the immunization by a singe injection of 0.1 cc could hardly give an immunization effect against infection, and the sera of thus treated mice showed no complement fixation with the ascites of guinea-pigs containing a large amount of rickettsiae. The mice which received 2 times of injection of 0.3 cc vaccine showed a marked resistance to the attack of the homologous strain, and also a slight resistance to that of the heterologous strain. Furthermore, the complement fixation reaction using thus immunized sera and the ascites antigen of the homologous strain gave the antibody titer of 10-40×, while in the reaction with the ascites antigen of the heterologous strain the antibody titer was 10-20×. When immunized by 3 times of injection of 0.3 cc vaccine, the resistance against the attack of the homologous strain was marked and was moderate against that of the heterologous strain, and the complement fixation reaction with the sera of thus immunized mice yielded the antibody titer of 80-160× against the ascites antigen of the homologous strain and 20-160× against that of the heterologous strain. When the sera of the mice immunized by the above-mentioned vaccines show the complement fixation titer of over 20× against the ascites antigen of the homologous strain, it may be assumed that the immunized mice possess a defensive power against the rickettsial infection.