In vitro studies on the ingestive and digestive capacities of rabbit mononuclear phagocytes from the peritoneal cavities of normal and previously immunized rabbits were made using avirulent, moderately virulent and highly virulent strains of Pasteurella tularensis. The types of serum (autologous, homologous and heterologous) present in 30% concentration and specific antibodies were not found to have any significant effects on the ingestive and intracellular destructive capabilities of these phagocytes on the three strains of the organism. Phagocytes from immune animals were found to have significantly increased intracellular destructive capacities in all cases as compared to those from normal animals. The following hypotheses have been defended: a) The degree of severity of P. tularensis infection in normal rabbits is dependent on the abilities of the phagocytic cells of the animals to destroy the organisms intracellularly; b) host resistance in acquired immunity to tularemia is primarily dependent on the altered phagocytic cells which have greatly enhanced intracellular destructive capacities; and c) humoral factors in acquired immunity to tularemia are secondary in importance.