Interleukin 2 (IL-2) stimulates the proliferation of T-cells both in vitro and in vivo. When murine or human peripheral blood (PB) or bone marrow (BM) mononuclear cells are incubated with IL-2 in vitro for 24 hours, cytotoxic T-cells are generated. If these activated cells are infused into mice, the enhanced cytotoxicity continues if low dose IL-2 is administered. This combination of administering activated cells with the subsequent low dose IL-2 infusion results in enhanced tumor cell destruction and improved survival rates in mice with acute myeloid leukemia. The encouraging results of these laboratory experiments prompted the initiation of phase I clinical trials in patients with refractory/relapsed hematologic malignancies and patients with breast cancer (Stages II-IV). Results from these trials demonstrate that stem cell transplantation with IL-2 activated stem cells (either PB or BM) with or without parenteral administration of IL-2 results in hematopoietic reconstitution with mild-to-moderate toxicities. This regimen also generates cutaneous and visceral autologous graft versus host disease (AuGVHD). The majority of our patients with relapsed/refractory hematologic malignancies or breast cancer developed either clinical and/or histological evidence of AuGVHD. Further studies are being conducted to determine if patients who develop AuGVHD experience improved disease-free survival from a possible autologous graft versus tumor (GVT) effect. Current laboratory evaluations include the elucidation of the pathogenesis of AuGVHD and molecular evaluation of the purging efficacy of IL-2.