The NHLBI Hematology Branch Stem Cell Transplantation program is exploring ways to make allogeneic transplantation safer and more widely applicable. Prior Hematology Branch transplant protocols have evaluated the strategy of using T cell depleted marrow transplants followed by delayed lymphocyte add-back to control or prevent GVHD while conserving useful donor immune function against residual leukemia and infectious agents. Over the past ten years, a number of increasingly efficient methods have been used to deplete T cells but retain stem cells, and we have shown the safety and utility of the delayed T cell add-back approach. We have also found a positive relationship between administration of higher CD34+ cell doses and outcome. Investigation of highly purified grafts with add-back of specific T cell populations is ongoing, and the ability to test new purification approaches and devices on clinical-scale PBSC products is critical to the continued development of new transplantation approaches in our program. This requires testing the approaches on G-CSF mobilized PBSCs collected by apheresis from healthy donors, since this is the cell source that will be used in all clinical allogeneic transplantation protocols in our program.