In mouse lines infected with R. tsutsugamushi, only some mice developed overt infection. This is most markedly manifested in random-bred and Swiss mice, causing high death rates and massive accumulation of rickettsiae in the peritoneum. Black mice respond to inoculation with asymptomatic infection with no microscopically detectable rickettsiae. The infection is accompanied in either case by an immunological response and persistence of the agent in mice. One mechanism of resistance to R. tsutsugamushi in some mouse lines is phagocytosis at the inoculation site of the main mass of rickettsiae by macrophages which digest the agent and retain their viability.