In addition to GVHD, allogeneic PBSCT has been limited by the toxicity associated with conventional myeloablative preparative regimens. Such regimens, which typically utilize total body irradiation (TBI) and high-dose chemotherapy, were once considered essential for the prevention of graft rejection. However, recent clinical studies have shown that non-myeloablative doses of fludarabine-based chemotherapy can result in alloengraftment. In murine models, we have demonstrated that severe host T cell depletion induced by combination fludarabine and cytoxan can prevent even fully-MHC mismatched marrow graft rejection. Although non-myeloablative regimens may reduce regimen-related toxicity, such transplants have been associated with a 30 to 40% incidence of severe acute GVHD that is similar to rates observed with myeloablative regimens. Because non-myeloablative regimens appear to be associated with reduced regimen-related toxicity, we have elected to conduct this phase I study of Th2 cells in the setting of an immunoablative (non-myeloablative) preparative regimen.