Acquired resistance to fully virulent strains of P. tularensis was passively transferred to normal mice by viable spleen cells or peritoneal leucocytes from donors that recovered from infection with an attenuated strain of P. tularensis. This passively transferred resistance was reflected in survival or delayed death after challenge of the recipients. The degree of passively transferred resistance was dependent upon the immune status of the donors and the number of viable immune cells transferred. There were indications that this resistance persisted only as long as the transferred tissues were compatible with the tissues of the recipient. The results support the hypothesis that immunity of mice against fully virulent strains of P. tularensis is associated with an altered state of the tissues.