Recently, we have demonstrated that antibodies that block the function of the β₂-integrin leukocyte function-associated antigen-1 (LFA-1) completely abrogate the rapid mobilization of hematopoietic progenitor cells (HPC) with colony-forming and radioprotective capacity induced by interleukin-8 (IL-8) in mice. These findings suggested a direct inhibitory effect of these antibodies on LFA-1–mediated transmigration of stem cells through the bone marrow endothelium. Therefore, we studied the expression and functional role of LFA-1 on murine HPC in vitro and in vivo. In steady state bone marrow ± 50% of the mononuclear cells (MNC) were LFA-1^neg. Cultures of sorted cells, supplemented with granulocyte colony-stimulating factor (G-CSF)/granulocyte-macrophage colony-stimulating factor (GM-CSF)/IL-1/IL-3/IL-6/stem cell factor (SCF) and erythropoietin (EPO) indicated that the LFA-1^neg fraction contained the majority of the colony-forming cells (CFCs) (LFA-1^neg 183 ± 62/7,500 cells v LFA-1^pos 29 ± 17/7,500 cells,P < .001). We found that the radioprotective capacity resided almost exclusively in the LFA-1^neg cell fraction, the radioprotection rate after transplantation of 10³, 3 × 10³, 10⁴, and 3 × 10⁴ cells being 63%, 90%, 100%, and 100% respectively. Hardly any radioprotection was obtained from LFA-1^pos cells. Similarly, in cytokine (IL-8 and G-CSF)–mobilized blood, the LFA-1^neg fraction, which comprised 5% to 10% of the MNC, contained the majority of the colony-forming cells, as well as almost all cells with radioprotective capacity. Subsequently, primitive bone marrow-derived HPC, represented by Wheat-germ-agglutinin (WGA)⁺/Lineage (Lin)⁻/Rhodamine (Rho)⁻ sorted cells, were examined. More than 95% of the Rho⁻ cells were LFA-1^neg. Cultures of sorted cells showed that the LFA-1^neg fraction contained all CFU. Transplantation of 150 Rho⁻ LFA-1^neg or up to 600 Rho⁻LFA-1^pos cells protected 100% and 0% of lethally irradiated recipient mice, respectively. These results show that primitive murine HPC in steady-state bone marrow and of cytokine-mobilized blood do not express LFA-1.