The ability of noncytolytic Lyt-1+,2- T cells immune to FBL-3 leukemia to effect eradication of disseminated FBL-3 was studied. Adult thymectomized, irradiated, and T-depleted bone marrow-reconstituted (ATXBM) B6 hosts were cured of disseminated FBL-3 by treatment with 180 mg/kg cyclophosphamide (CY) and adoptively transferred Lyt-1+,2- T cells obtained from congenic B6/Thy-1.1 donors immune to FBL-3. Analysis of the T cell compartment of ATXBM hosts treated and rendered tumor-free by this therapy revealed that the only T cells present in the mice were donor-derived Lyt-1+,2- T cells. In vitro stimulation of these T cells with FBL-3 tumor cells, which express class I but no class II major histocompatibility complex antigens, induced lymphokine secretion, but did not result in the generation of cytotoxic T lymphocytes (CTL). Thus, in a setting in which mice lack Lyt-2+ T cells, and in which no CTL of either host or donor origin could be detected, immune Lyt-1+,2- T cells, in conjunction with CY, mediated eradication of a disseminated leukemia. The results suggest that delayed-type hypersensitivity responses induced by immune T cells represent a potentially useful effector mechanism for in vivo elimination of disseminated tumor cells.