Microcinematography, cytological and cytochemical studies revealed new features of D. sibericus, D. murinus, and R. tsutsugamushi biology, mainly a very active movement in the cell. In contrast to D. sibericus, development of D. murinus and R. tsutsugamushi in cells is characterized by severe irritation of the infected cells early in the infection, a greater speed of movement of rickettsiae and their active release from the affected cells, as well as the development of necrobiotic changes in the latter. The mitotic activity of the cells increased early in cultures infected with Cox. burneti, R. prowazeki, D. sibericus, and T. tsutsugamushi. Subsequently, cell nuclei shrink and different cytotic changes develop in various rickettsia. Studies on the metabolism of the affected cells revealed activation of nucleic metabolism and redox enzymes associated with mitochondria in the course of the first 3 days followed by a low activity in 5 postinfection days. The lysosomal apparatus of the cells underwent drastic changes and the activity of acid phosphatase increased markedly with further elevation in the permeability of lysosomal membranes.