Patients who reject allogeneic blood or marrow grafts after non-myeloablative conditioning sometimes develop delayed or sustained clinical disease responses, raising the possibility that a transient graft-versus-host reaction rekindles dormant anti-tumor immunity by recipient T cells. More specifically, we postulate that endogenous tumor-specific CD8+ T cells are not irreversibly tolerant of a growing tumor, and can be activated to effector function by tumor antigen recognition in the presence of adoptively transferred CD4+ T cell help. We have tested this hypothesis by examining the response of influenza hemagglutinin (HA)-specific “endogenous” CD8+ T cells in HA-tumor-bearing mice treated with cyclophosphamide (Cy) and adoptive transfer of allogeneic or tumor-specific CD4+ T cells, with or without concomitant tumor vaccine administration. In these models, this combination therapy induced the clonal expansion of endogenous, tumor-specific CD8+ T cells, their secretion of interferon gamma, and in vivo cytolytic activity against HA-expressing cells. In separate experiments, adoptively transferred, major histocompatibility complex-mismatched CD4+ T cells induced the activation of recipient, CD11c+ dendritic cells, as demonstrated by enhanced expression of the costimulatory molecules CD80 and CD86. The administration of Cy followed by infusion of allogeneic T cells, with or without tumor vaccine administration, also induced regression of B cell leukemia/lymphoma or established prostate cancer. These results highlight the potential cooperation of donor CD4+ T cells and recipient CD8+ T cells in transient or stable mixed hematopoietic chimeras, and open new avenues for the immunotherapy of cancer in the clinic.