The investigators propose a nonrandomized, Phase I study to assess the safety of infusion of NK cells that will be selected from sibling donors and infused to patients with hematological malignancies early following allogeneic stem cell transplantation.

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.

Inclusion Criteria:

1. Patients undergoing an allogeneic HSCT from a sibling donor, as treatment for a hematological malignancy. The conditioning regimen, and in particular whether ablative or non ablative, will not be considered in the criteria for recruitment
2. Patient and donor Age >18 years
3. Patients and donors must have signed an informed consent form
4. The donor must be willing and capable of donating lymphocytes for NK selection using apheresis techniques
5. Donor must be fit to undergo leukapheresis

Exclusion Criteria:

1. Life expectancy < 3 months
2. ECOG performance status 3 or 4
3. Uncontrolled intercurrent illness including, but not limited to ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, life threatening cardiac arrhythmia
4. Patients will not be eligible if they receive in vivo T depletion with ATG, ALG or campath-1H
5. HIV-positive patients
6. Psychiatric illness/social situations that would limit compliance with study requirements and ability to comprehend the investigational nature of the study and provide informed consent