In the setting of allogeneic bone marrow transplantation (alloBMT), donor T cells mediate a beneficial graft-versus-leukemia (GVL) effect and prevent marrow rejection; however, donor T cells can also generate graft-versus-host disease (GVHD). We have identified that donor CD8+ T cells of Tc2 phenotype, defined by their secretion of type II cytokines and their mediation of target cell lysis primarily via the perforin pathway, are capable of mediating a GVL effect and preventing marrow rejection with limited GVHD. Current efforts are evaluating the use of such Tc2 cells in murine models of non-myeloablative alloBMT. In addition, pilot clinical trials evaluating the effect of donor Tc2 cells in the setting of human alloBMT are being developed. The overall goal of this work is to improve the anti-leukemic effects of alloBMT, to reduce the morbidity and mortality associated with GVHD, and to extend the application of alloBMT to those patients lacking an HLA-matched donor.In the autologous setting, T cells may also play a role in mediating anti-leukemic effects. Murine models are being developed to evaluate the role of the fas cytolytic pathway in T cell-mediated syngeneic GVL effects. In addition, studies in CLL patients are evaluating the differential role of type I versus type II cytokines on CLL cell utilization of the fas pathway. These murine and pre-clinical human studies may provide a basis for modulation of the fas pathway for the treatment of leukemia. In addition, we have developed methodologies to purge CLL cells from peripheral T cell populations. Protocols to evaluate whether such autologous T cells might improve immune recovery after purine analog-based chemotherapy are being developed.