Adoptive transfer of T lymphocytes harvested from spleen of Rickettsia conorii-infected DBA/2 mice to intact and cyclophosphamide- (CPA-) pretreated syngeneic mice protected the latter from lethal infection caused by R, conorii. Protection from infection was not observed in recipient mice given immune serum or B lymphocytes and macrophages from the spleens of convalescent mice. The protective effect was most pronounced after intravenous transfer of lymphocytes obtained from donor mice on day 14 post infection. Lethal infection was not prevented by transfer of lymphocytes harvested on day 50 p. i., although at this interval the donor mice were still resistant to reinfection with F. conorii.