Bone marrow transplantation (BMT) is indicated for selected genetic and hematologic diseases and for hematopoietic reconstitution in cases of iatrogenic or accidental ablation of BM.’-5 Additionally, allogeneic BMT may contribute to the therapy of leukemia through graft-versus-leukemia effects (GVL), which are associated with, but possibly independent of, chronic graft-versus-host disease (GVHD). 6-15 HLA identical sibling donors offer the best graft compatibility, but HLA-identical unrelated donors may also provide acceptable grafts. 16 Hence, the organization, in the United States, Europe, and elsewhere, of large registries of HLA-typed individuals who volunteer to donate BM for recipients lacking HLA-identical sibling donors. 17-20 Placental blood, as an alternative source of hematopoietic stem cells for BM reconstitution, has recently been shown to yield successful sibling-donor placental blood “grafts” in~ hildren.~’-~~ Since, furthermore, the numbers of stem/progenitor cells in placental blood25-2s are in the range associated with successfully transplanted adult BM, 29-31 placental blood has the potential to overcome some of the limitations of the current system of registries for unrelated marrow donor procurement,“Banks” of cryopreserved placental bloods would not depend, for example, on the recruitment and continued collaboration of large numbers of volunteer potential donors and on compensating for the unavoidable attrition caused by retired volunteers. Systematic studies of the feasibility of using banked placental blood for BM reconstitution of unrelated recipients on a large scale seem, therefore, timely and warranted. At least three such studies have been proposed, including our