We compared the chemotactic responsiveness of different subsets of human B lineage cells to stromal derived factor-1 (SDF-1). High percentages (30–40% of input) of purified bone marrow progenitors including non-B lineage progenitors, pro-B cells, and pre-B cells migrated to SDF-1α, demonstrating that SDF-1 is an efficacious chemoattractant of these cells. Pro-B cells responded optimally to a lower concentration of SDF-1 than other subsets, demonstrating that SDF-1 is a more potent chemoattractant of this subset. A lower percentage (10–15% of input) of mature B lymphocytes migrated to SDF-1α than pro-B cells, demonstrating that responsiveness of B lineage cells to SDF-1 decreases during differentiation. Inhibition by anti-CXCR4 mAb demonstrated that migration of B lineage cells to SDF-1 was completely dependent on CXC chemokine receptor-4 (CXCR4). Mature B cells expressed higher levels of CXCR4 receptors than uncommitted progenitors and pro-B cells, despite differences in responsiveness to SDF-1. CXCR4 receptors expressed by unresponsive and SDF-1-responsive B cells bound SDF-1a with similar affinities (K  _D= 1.7–3.3 × 10⁻⁹ M). Therefore, elements downstream from CXCR4 appear to regulate responsiveness of B cells to SDF-1. We speculate that SDF-1 and CXCR4 direct migration of progenitor cells in microenvironments that promote B lymphopoiesis. J. Leukoc. Biol. 66: 667–673; 1999.