The study identifies a highly efficacious lymphocyte chemoattractant, stromal cell-derived factor 1 (SDF-1), produced by murine bone marrow stromal cells (MS-5). SDF-1 attracts 10-fold more lymphocytes in vitro compared to other known lymphocyte chemoattractants and is both highly efficacious and potent in vivo. SDF-1 acts on lymphocytes and monocytes but not neutrophils, and induces intracellular actin polymerization, a prerequisite for cell motility. SDF-1 is constitutively expressed in a broad range of tissues, suggesting a role in immune surveillance and basal extravasation of lymphocytes and monocytes rather than in inflammation. The evolutionary and sequence homology properties of SDF-1 are unique among chemokines, and it may represent a primordial chemokine ancestor.The study identifies a highly efficacious lymphocyte chemoattractant, stromal cell-derived factor 1 (SDF-1), produced by murine bone marrow stromal cells (MS-5). SDF-1 attracts 10-fold more lymphocytes in vitro compared to other known lymphocyte chemoattractants and is both highly efficacious and potent in vivo. SDF-1 acts on lymphocytes and monocytes but not neutrophils, and induces intracellular actin polymerization, a prerequisite for cell motility. SDF-1 is constitutively expressed in a broad range of tissues, suggesting a role in immune surveillance and basal extravasation of lymphocytes and monocytes rather than in inflammation. The evolutionary and sequence homology properties of SDF-1 are unique among chemokines, and it may represent a primordial chemokine ancestor.