This is a pilot study which began with a plan to enroll 50 patients (20 related and 30 unrelated donor transplants) with hematological malignancy assessing the safety and efficacy of a minimally myelosuppressive regimen with Pentostatin and low-dose total body irradiation (TBI) followed by allogeneic peripheral blood stem cell transplantation (alloPSCT). Patients with persistent or progressive malignancy after transplantation will be treated with GM-CSF (cytokine therapy) to assess its toxicity and potential therapeutic efficacy. Patients with persistent or progressive disease who fail or do not qualify for the cytokine therapy portion of the study will become candidates for donor leukocyte infusions.