Background: Certain blood cancers can be treated with blood or bone marrow transplants. Sometimes the donor cells attack the recipient's body, called graft-versus-host disease (GVHD). The chemotherapy drug cyclophosphamide helps reduce the risk and severity of GVHD. Researchers want to learn if using a lower dose of cyclophosphamide may reduce the drug's side effects while maintaining its effectiveness. Such an approach is being used in an ongoing clinical study at the NIH with promising results, but this approach has not been tested for transplants using lower doses of chemotherapy/radiation prior to the transplant.

Objective: To learn if using a lower dose of cyclophosphamide will help people have a successful transplant and have fewer problems and side effects.

Eligibility: Adults ages 18-85 who have a blood cancer that did not respond well to standard treatments or is at high risk for relapse without transplant, and their donors.

Design: Participants may be screened with the following: Medical history Physical exam Blood and urine tests Heart and lung tests Body imaging scans (they may get a contrast agent) Spinal tap Bone marrow biopsy Participants will be hospitalized for 4-6 weeks. They will have a central venous catheter placed in a chest or neck vein. It will be used to give medicines, transfusions, and the donor cells, and to take blood. In the week before transplant, they will get 2 chemotherapy drugs and radiation. After the transplant, they will get the study drug for 2 days. They will take other drugs for up to 2 months. Participants must stay near NIH for 3 months after discharge for weekly study visits. Then they will have visits every 3-12 months until 5 years after transplant. Participants and donors will give blood, bone marrow, saliva, cheek swab, urine, and stool samples for research.