SDF-1 is a member of the CXC chemokines. In contrast to other chemokines that are induced by inflammation, SDF-1 is constitutively produced by stromal cells. In order to investigate the physiological roles of SDF-1, we constructed a fusion protein, SDF-1-Cgamma1, composed from murine SDF-1alpha and the constant region of human IgG. SDF-1-Cgamma1 stained EL-4 T lymphoma cells and the staining was blocked by rhSDF-1beta. The expression levels of SDF-1R altered along with the T cell maturation. Most c-kit+ hematopoietic precursors in fetal liver in gestational day (GD) 14.5 embryo were SDF-1R-, while c-kit+ double-negative (DN) thymocytes in the embryo were positive for SDF-1R. The receptor expression increased along with T cell maturation up to double-positive (DP) cell stage. Interestingly, SDF-1R expression was down-modulated after positive selection; CD69+CD3hi DP and CD3hi single-positive thymocytes were SDF-1R-/lo. Northern blot analysis demonstrated that SDF-1 and CXCR4 mRNAs were abundantly expressed in the thymuses of embryo and adult mice. These results demonstrate that SDF-1R expression is involved in T cell development in the thymus, particularly in positive selection.