Allogeneic hematopoietic stem cell transplantation (HSCT) is a very effective treatment for a number of hematological malignancies but relapse remains a major problem, especially in patients with high risk disease. Natural killer (NK) cells are immune cells that recognize and kill virally infected cells and tumor cells. NK cells are identified by the expression of the CD56 surface antigen and the lack of CD3. Their ability to kill tumor cells makes them promising to evaluate as effector cells for immunotherapy.