Both SDF-1 and CXCR4 disruption are lethal to mice at the embryonic stage and cause abnormalities in B lymphopoiesis, myelopoiesis, cardiogenesis, vasculogenesis, and cerebellar development. To investigate the role of SDF-1 and CXCR4 in hematopoiesis during the adult stage, mice reconstituted with bone marrow–derived hematopoietic progenitor cells transduced with either the SDF-1 or a genetically modified SDF-1–intrakine gene using a retroviral expression vector were analyzed. Flow cytometric (FCM) analysis showed a dramatic reduction of CXCR4 expression on the cells of intrakine-transduced mice, whereas CCR7 and CCR1 expression was unchanged or marginally decreased on splenocytes. Migration of splenocytes and bone marrow cells to SDF-1 was markedly suppressed in intrakine-transduced mice. FCM analysis of bone marrow cells of intrakine-transduced mice exhibited decreased numbers of pro-B (B220⁺ CD43⁺), pre-B (B220⁺CD43⁻), and immature B (B220⁺IgM⁺) cells and a decreased number of granulocytes/myeloid (Gr1⁺ CD11b⁺) cells. Impaired B lymphopoiesis and myelopoiesis in intrakine-transduced mice were confirmed by an in vitro colony-forming assay of bone marrow cells. In contrast, B lymphopoiesis and myelopoiesis were enhanced in SDF-1–transduced mice. Interestingly, T-cell maturation in the thymus was impaired both in intrakine- and SDF-1–transduced mice, suggesting that SDF-1 and CXCR4 play an important role in T lymphopoiesis as well as in B lymphopoiesis and myelopoiesis in adults. These results demonstrate an essential role of CXCR4 and its ligand SDF-1 in adult hematopoiesis, and they indicate the intrakine method as a powerful tool for functional analysis of chemokines/chemokine receptors in vivo and as a potential therapeutic approach for acquired immunodeficiency syndrome.