Rickettsia prowazekii was assessed for in vitro susceptibility to phagocytosis by rabbit polymorphonuclear leukocytes. [alpha-32P]adenosine triphosphate-labeled rickettsiae were used to determine phagocytosis and adsorption quantitatively. R. prowazekii was less susceptible to phagocytosis than were Escherichia coli and Neisseria gonorrhoeae. Although R. prowazekii was similar to E. coli in susceptibility to superoxide and activated halide, few phagocytized rickettsial cells were inactivated after being ingested by polymorphonuclear leukocytes, and rickettsiae were observed free in polymorphonuclear leukocyte cytoplasm. At low ratios of rickettsiae to polymorphonuclear leukocytes, PMN phagocytosis increased as a linear function of time, but at high ratios (multiplicity of infection, 50) rickettsiae were phagocytized during only the first 10 min of incubation. Polymorphonuclear leukocytes were damaged in the presence of high rickettsial multiplicities such that they released lactate dehydrogenase into the medium and lost the ability to phagocytize both rickettsiae and E. coli. The amount of leukotoxic activity in a given rickettsial sample correlated with the relative hemolytic activity of that sample. The rickettsial leukotoxin was probably not a soluble product, was active in the absence of phagocytosis, and was inhibited by inactivation of the rickettsiae or by incubation at 4 degrees C.