Host mechanisms of resistance to Pasteurella tularensis infections in white mice, guinea pigs and rabbits were studied by using passively transferred phagocytic cells and sera to protect the challenged animals. Homologous and heterologous cells and sera from normal and previously immunized (recovered) animals were used. Three strains of the pathogen, varying in virulence for the three animal species, were used as challenge organisms. It was found that specific antibodies, whether from a homologous or heterologous source, were capable of increasing the survival times of challenged animals, while phagocytes from immune animals (homologous and heterologous) increased the survival times and decreased the mortality rates. This latter phenomenon was also evident using normal cells from a naturally resistant species to protect a naturally susceptible species. It was concluded that cellular factors were of critical importance in acquired immunity to tularemia while specific antibodies per se were of less significance.