The chemokine stromal cell-derived factor-1 (SDF-1) and its receptor CXCR-4 (fusin, LESTR) are likely to be involved in the trafficking of hematopoietic progenitor and stem cells, as suggested by the reduced bone marrow hematopoiesis in SDF-1–deficient mice and the chemotactic effect of SDF-1 on CD34⁺ progenitor cells. Migration of leukemic cells might also depend on the expression of chemokine receptors. Therefore, we analyzed expression of CXCR-4 on mobilized normal CD34⁺ progenitors and leukemic cells. In addition, SDF-1–induced transendothelial migration across a bone marrow endothelial cell layer was assessed in vitro. By flow cytometry, CXCR-4 was found to be expressed in significant amounts on circulating CD34⁺ hematopoietic progenitor cells, including more primitive subsets (CD34⁺/CD38⁻ and CD34⁺/Thy-1⁺ cells). In accordance with the immunofluorescence data, CD34⁺ progenitors efficiently migrated across endothelium in response to SDF-1 containing conditioned medium from the stromal cell line MS-5. Leukemic blasts (mostly CD34⁺) from patients with acute myeloblastic leukemia (AML) expressed variable amounts of CXCR-4, which was functionally active, as demonstrated by a positive correlation between the SDF-1–induced transendothelial migration and the cell surface density of CXCR-4 (r = 0.97). Also recombinant SDF-1β induced migration of CXCR-4–positive leukemic blasts. The effect of both conditioned medium and recombinant SDF-1 was inhibited by a CXCR-4 blocking antibody. In contrast, CD34⁺ leukemic cell lines (KG1, KG1a, Kasumi-1, MOLM-1) expressed low levels or were negative for CXCR-4, and did not migrate. By reverse transcriptase-polymerase chain reaction (RT-PCR), however, basal levels of CXCR-4 mRNA were also detected in all leukemic cell lines. We conclude that CXCR-4 is expressed on CD34⁺cells including more primitive, pluripotent progenitors, and may therefore play a role in the homing of hematopoietic stem cells. CXCR-4 expressed in variable amounts on primary AML leukemic cells is functionally active and may be involved in the trafficking of malignant hematopoietic cells.