It is well-known fact that we can see the cross-immunization with guinea pigs between R. pmwazeki and R. mooseri. But we cannot find any reports concerning the experiments about the grades of immunization by R. prowazeki or R. mooseri. The author took the method to immunize the guinea pigs through the subcutaneous route. The antigens used were formol vaccines made from Citellus mongolicus infected with R. prowazeki and R. mooseri, Cox-Craigies vaccine and a small, but enough amount of viruses to cause latent infection. Challenge tests against the immunized guinea pigs were performed through intracardial, intratesticular and subcutaneous route. The guinea pigs, immunized by various kinds of antigen, were challenged through various routes by the rickettsial strains. The results of this cross-immunization were as follows: 1. Formol vaccine of R. prowazeki proved to have the protective activity against challenges by homologous and heterologous rickettsial strains regardless of the ways of their passage. 2. Formol vaccine of R. mooseri proved to have complete protective activity against homologous strain, and also have partial protective activity against heterologous strain which had been generated through intraperitoneal route, but only a little protective activity against heterologous strain which had been generated through intracardial route. 3. Concerning the protective activities, induced from latent infection of R. prowazeki and R. mooseri, the guinea pigs immunized by R. prowazeki showed better protective activity against homologous strain than against heterologous one. 4. The protective activity induced from Rickettsia prowazeki had a tendency to be inferior against R. prowazeki strain, which had been generated successively through intracardial route, to the strain generated through the other routes. 5. Conclusively, to see the protective activities of the immunized guinea pigs against rickettsial strains used for challenge, we must know and regard about the strains how had been their successive generation made and what routes had been taken to innoculate.