Rituximab (Rituxan) for the Prevention of EBV-LPD Epstein Barr Virus (EBV) Lymphoproliferative Disorder Post T Cell Depleted Unrelated and HLA Mis-matched Related HSCT

Inclusion Criteria:

* Patient must be a recipient of aT cell depleted unrelated or HLA mis-matched related HSCT for the treatment of a malignancy or immunodeficiency disease.
* Patients must have an ANC \> or = to 1500 cells/ul on the day of first treatment.
* Patients with acute or chronic leukemia, or MDS prior to transplant must be in remission defined as \<5% blasts in the bone marrow.
* Patient with must be in remission.
* Patient must be Hepatitis B surface antigen negative pre transplant.
* Patients must have adequate cardiac function defined as a left ventricular ejection fraction at rest of \>50% documented pre-transplant.
* Patient may be of either gender and of any ethnic background.
* Patient may be of any age. There is no upper age restriction.
* Patients or their guardians must be able to understand the nature and risk of the proposed study and be able to sign consent.

Exclusion Criteria:

* Karnofsky score \<70%
* Female patients who are pregnant or lactating.
* Evidence of EBV-LPD or circulating EBV copy number \>1000.
* Active uncontrolled bacterial or fungal infection.
* Prior history of Hepatitis B infection or Hepatitis B surface antigen positivity pre transplant.
* HIV-1,2 sero-positive patients.
* Patients or guardians not signing informed consent.
* Patients with prior allergic reaction to Rituximab or other murine monoclonal antibody.
* Patients taking other investigational agents under another protocol unless discussed and approved in advance by Genentech and the IDEC Therapeutic Director.