The nature and exact function of selectin ligands involved in hematopoietic progenitor cell (HPC) homing to the bone marrow (BM) are unclear. Using murine progenitor homing assays in lethally irradiated recipients, we found that the P-selectin glycoprotein ligand-1 (PSGL-1) plays a partial role in HPC homing to the BM (a reduction of about 35% when the P-selectin binding region is blocked). Blockade of both PSGL-1 and alpha4 integrin did not further enhance the effect of anti-alpha4 integrin (a reduction of about 55%). We suspected that E-selectin ligands might contribute to the remaining homing activity. To test this hypothesis, HPC homing assays were carried out in E-selectin-deficient recipients and revealed a profound alteration in HPC homing when E-selectin and alpha4 integrin were inactivated (> 90% reduction). Competitive assays to test homing of long-term repopulating stem cells revealed a drastic reduction (> 99%) of the homed stem cell activity when both alpha4 integrin and E-selectin functions were absent. Further homing studies with PSGL-1-deficient HPCs pretreated with anti-alpha4 integrin antibody revealed that PSGL-1 contributes to approximately 60% of E-selectin ligand-mediated homing activity. Our results thus underscore a major difference between mature myeloid cells and immature stem/progenitor cells in that E-selectin ligands cooperate with alpha4 integrin rather than P-selectin ligands.