Injection of C3H × CBA hybrid lymphocytes into CBA mice specifically reduces the pool of host T cells which are reactive against the Mls antigen determined by the C3H genome. Since C3H × CBA lymphocytes are triggered to cell division in the spleens of irradiated CBA mice we have now examined if such activated cell populations exhibit any effector functions against CBA lymphocytes which are reactive against C3H-determined antigens. It was observed that such ‘educated’ cell populations were unable to significantly kill CBA lymphocytes which are triggered to cell division in C3H × CBA hosts or CBA lymphocytes which can inhibit proliferation of transplanted C3H × CBA bone marrow cells. Moreover, there was no evidence that such ‘educated’ cell populations can specifically kill anti-C3H-reactive CBA lymphocytes in vitro or damage CBA bone marrow cells in vivo. Thus, these results do not demonstrate that C3H × CBA lymphocytes acquire any effector functions against CBA T cells which are reactive against C3H-determined antigens.