Histological examinations of mice infected with either a lethal (Karp) or a self-limitating (Gilliam) strain of Rickettsia tsutsugamushi were performed. Tingible body macrophages in the spleen and necrotizing lymphadenitis in regional lymph nodes were prominent only in the former. Apoptotic legions in the lymphocytes of these organs were clearly demonstrated by histochemical and electron microscopical examinations.