Adoptive transfer of protective immunity to an aerogenic infection with the facultative intracellular bacterium Mycobacterium tuberculosis was mediated by a population of T cells acquired in the spleens of donor mice at the height of the primary cell-mediated immune response to an immunizing infection with M. bovis bacillus Calmette-Guerin. Successful adoptive immunotherapy was ablated by prior exposure of immune donor cells to ionizing radiation or by treatment of these cells with antibody raised against the Ly-2 marker. In contrast, however, the capacity of immune donor cells to passively transfer delayed-type hypersensitivity (DTH) responses to tuberculin was unaffected by prior treatment with antibody to Ly-2, but was completely ablated by treatment by antibody to Ly-1. These results indicate, therefore, that DTH and protective anti-tuberculous immunity are dissociable phenomena, mediated by separate populations of T lymphocytes.