Recipients of autologous BMT demonstrate clinically significant immune deficiency, particularly involving the T lymphocytes. While quantitatively the immune system generally returns to normal during the first 3 months, there is a prolonged delay in the recovery of qualitative immune functions. T cell proliferation is impaired immediately after transplantation and slowly recovers over a period of more than 1 year. In addition, a defect has been documented in IL-2 producing cells and may be of major importance in the pathophysiology of this immunodeficiency. However, post-ABMT, PHA-stimulated T cells are TAC+ and are able to respond to exogenous IL-2 in vitro. Very early after ABMT, NK and LAK activities of PBMC normalize but are significantly increased in vitro by IL-2. On this basis, a clinical assessment of rIL-2 administration on the immunological reconstitution of ABMT patients and as consolidation immunotherapy against minimal disease has been initiated in a phase I/II study.