The phagocytosis and subsequent degradation of phase I and II Coxiella burnetii by macrophages obtained from immune and nonimmune guinea pigs were compared. Phase I rickettsiae were more resistant to phagocytosis than were phase II organisms. There was no significant difference in the percentage of phagocytosis of either phase of rickettsiae by macrophages from immune or nonimmune animals. After ingestion, phase I and II organisms pretreated with normal serum multiplied and destroyed normal macrophages as well as macrophages obtained from guinea pigs immunized with phase II rickettsiae. In contrast, only phase I organisms were degraded by macrophages from phase I-immunized animals in the presence of normal serum. Immune serum rendered rickettsiae more susceptible to phagocytosis and also potentiated the destruction of organisms by all types of macrophages. The specificity of macrophages from phase I animals to degrade only phase I rickettsiae was demonstrated by the ability of Rickettsia rickettsii to replicate in these macrophages.