Macrophages obtained from the abdominal cavities and spleens of DBA/2 mice and guinea pigs convalescent after Rickettsia conorii infection digested the rickettsiae in vitro more actively than those from uninfected animals. The activation of macrophages was manifested by their capacity to inhibit replication of the rickettsiae and to digest them as well as by their resistance to the toxic effect of rickettsiae. Visual observations and bioassays showed that a portion of the rickettsial population survived in the culture producing no toxic effect on the cells which could be readily passaged.