The β2 integrins leukocyte function antigen-1 (LFA-1, CD11a) and macrophage antigen-1 (Mac-1, CD11b) have been reported to play a role in the attachment of CD34⁺ cells to stromal cells in the bone marrow. When administered prior to interleukin-8 (IL-8), anti–LFA-1 antibodies completely prevent the IL-8–induced mobilization of hematopoietic stem cells in mice. Here, we studied the role of anti-β2 integrin antibodies in granulocyte colony-stimulating factor (G-CSF)–induced mobilization of hematopoietic progenitor cells. Administration of antibodies against the α chain of LFA-1 or against the α chain of Mac-1 followed by daily injections of G-CSF for more than 1 day resulted in a significant enhancement of mobilization of hematopoietic progenitor cells when compared with mobilization induced by G-CSF alone. Also, the number of late (day 28) cobblestone area–forming cells in vitro was significantly higher after mobilization with anti–LFA-1 antibodies followed by 5 μg G-CSF for 5 days than with G-CSF alone (119 ± 34 days vs 17 ± 14 days), indicating mobilization of repopulating stem cells. Pretreatment with blocking antibodies to intercellular adhesion molecule-1 (ICAM-1; CD54), a ligand of LFA-1 and Mac-1, did not result in an effect on G-CSF–induced mobilization, suggesting that the enhancing effect required an interaction of the β2 integrins and one of their other ligands. Enhancement of mobilization was not observed in LFA-1–deficient (CD11a) mice, indicating that activated cells expressing LFA-1 mediate the synergistic effect, rather than LFA-1–mediated adhesion.