It has been proposed that platelets possess a P_2X1-purinoceptor-like ligand-gated cation channel, through which Ca²⁺enters platelets from the extracellular medium upon ADP or ATP stimulation. In this paper we describe the cloning of human P_2X1-specific cDNA from human platelets, K562 and human erythroleukaemic cell lines. Sequence analyses of these cDNAs show 100% nucleotide sequence identity with that of human P_2X1cloned from urinary bladder. Western blotting of platelet lysates separated by SDS-PAGE and probed with anti-P_2X1IgG shows the expected protein with a molecular mass of 60kDa and a second protein of 45kDa. These data confirm that platelets possess at least two distinct purinoceptors: a P_2Tpurinoceptor which mediates platelet aggregation, inhibition of adenylate cyclase, and release of intracellular Ca²⁺stores and a platelet P_2X1purinoceptor which upon ATP and ADP stimulation mediates the rapid entry of extracellular Ca²⁺into platelets.