Gene therapy for inherited disorders of blood cells will require both efficient methods for stable gene transfer and nonablative bone marrow conditioning regimens to allow engraftment of modified hematopoietic progenitor cells (HPCs). We have used a sensitive murine system for detecting HPC engraftment using congenic C57BL/6 mice that differ at the Ly5 locus, which encodes the leukocyte common antigen. The system relies on the ability of monoclonal antibodies with specificity for Ly5.1 and Ly5.2 (revised nomenclature: CD45.1 and CD45.2, respectively) to distinguish donor and recipient peripheral blood leukocytes after transplantation of purified Sca-1+ bone marrow-derived HPCs. No detectable engraftment occurred in nonirradiated recipient mice, even when as many as 2.0 x 10(6) SCa-1+HPCs were transplanted. However, in mice receiving total body irradiation (TBI), engraftment increased as a function of pretransplantation radiation dose, number of transplanted cells, and time after transplantation. Moreover, mice receiving either granulocyte colony-stimulating factor (G-CSF) or G-CSF+ stem cell factor before low-dose TBI (160 cGy) exhibited a marked increase in engraftment compared with mice receiving a vehicle control before low-dose TBI (18.9% and 20.6% v 5.6% at a 1 month, respectively; 29% and 35% v 15.1% at 4 months, respectively). Use of growth factor pretreatment even allowed TBI doses as low as 30, 70, or 120 cGy to achieve significant engraftment of donor progenitors (0.3%, 1.5%, and 6.8% at 1 month, respectively; 1.7%, 5.8%, and 13.9% at 4 months, respectively). All animals remained healthy during the observation periods. Thus, growth factor preconditioning of the recipient followed by low-dose TBI may provide an optimal balance between safety and efficacy in achieving required levels of engraftment for gene therapy of blood disorders.