—Chemokines are chemotactic cytokines that activate and direct the migration of leukocytes. However, their role in modulating platelet function has not been shown. We studied the direct effect of chemokines on human platelets and found that of the 16 tested only stromal cell–derived factor (SDF)-1 induced platelet aggregation, accompanied by a rise in intracellular calcium. Platelets expressed the SDF-1 receptor, CXCR4, and an antibody to CXCR4 and pertussis toxin inhibited SDF-1–induced platelet aggregation, confirming that this effect is mediated through CXCR4, a Gαi-coupled receptor. SDF-1–induced platelet aggregation was also inhibited by wortmannin, LY294002, and genistein, suggesting that phosphatidylinositol 3–kinase and tyrosine kinase are likely involved in SDF-1–induced platelet aggregation. Because chemokines are produced from multiple vascular cells and atherosclerotic vessels are prone to develop platelet-rich thrombi, we examined the expression of SDF-1 in human atheroma. SDF-1 protein was highly expressed in smooth muscle cells, endothelial cells, and macrophages in human atherosclerotic plaques but not in normal vessels. Our studies demonstrate a direct effect of a chemokine in inducing platelet activation and suggest a role for SDF-1 in the pathogenesis of atherosclerosis and thrombo-occlusive diseases.