Extracellular adenosine triphosphate (ATP) and adenosine diphosphate (ADP) activate multiple types of P2-nucleotide receptors expressed in platelets or leukocytes. Electrophysiological and biochemical studies have indicated expression of the P2X₁ receptor, an ATP-gated cation channel, in human and rat platelets, rat basophilic leukemia (RBL) cells, and phorbol myristate acetate (PMA)-differentiated HL-60 myeloid cells. Although these findings suggest that P2X₁ receptors are present in both blood leukocytes and blood platelets, the relative levels of P2X₁receptor expression and function in human blood leukocytes and platelets have not been directly characterized. On the basis of both immunoblot analysis and functional assays of P2X₁receptor-mediated ionic fluxes, we report that there is significant expression of P2X₁ receptors in human platelets, but not in neutrophils, monocytes, or blood lymphocytes. Thus, unlike platelets and myeloid progenitor cell lines, fully differentiated human blood leukocytes do not express functionally significant numbers of P2X₁ receptors, suggesting the downregulation of P2X₁ receptor gene expression during the differentiation of phagocytic leukocytes. By contrast, P2X₁ receptor expression is strongly maintained during megakaryocytic differentiation and platelet release. Immunoblot analysis indicated that the platelet P2X₁ receptor migrates as an approximately 60-kD protein during SDS-electrophoresis under reducing or nonreducing conditions. Treatment of platelet membranes with endoglycosidase-F causes the P2X₁ receptor band to migrate as a 46-kD protein, verifying the highly glycosylated nature of the mature receptor protein. Additional studies of nucleotide-induced changes in Ca²⁺influx/mobilization demonstrated that the platelet P2X₁receptors are pharmacologically distinct from the well-characterized ADP receptors of these cells. This finding suggests a unique role for these ATP-gated ion channels during hemostasis or thrombosis.