Summary: Following infection with R. akari or R. australis, Swiss mice developed species-specific antibody which, unlike guinea pig convalescent sera, did not fix complement with the soluble, cross-reacting antigen of heterologous rickettsias of the spotted fever group. The EM rickettsia, a newly described agent recovered from ticks from eastern Montana, also produced highly specific CF antibody in mice. Cross-reactions were not observed between the EM agent and any of the members of the spotted fever group tested. Species-specific antibody was produced in mice by other agents of the group, i.e., R. rickettsi, R. conori and R. siberica. Yet, because of the extremely large dose of these rickettsias required and the erratic results obtained, these studies are regarded as preliminary only. Toxin-neutralizing, but not CF, antibody could be detected in sera of mice inoculated with relatively low doses of live or inactivated R. rickettsi.