Multiple sclerosis (MS) is an autoimmune inflammatory disease of the central nervous system characterized by acute relapses followed by remissions, which become incomplete and lead to accumulation of disability. Recent studies have shown an increase in circulating microparticles derived from activated platelets in MS [1], but it is not known how this relates to the pathophysiology of acute relapses or disability progression. In a case-control genetic association study of MS we examined twelve functional polymorphisms of P2X7. One variant, rs28360457, coding for the loss of function Arg307Gln was associated with twofold protection against MS (OR 0.57, p=0.0000024) while the gain of function variant of P2X7, Ala348Thr, was associated with an increased risk of MS (OR 1.10, p=0.0006) [2]. Our aim is to find if P2X7 is expressed on circulating platelets and whether it plays a role in the generation of circulating platelet-derived microparticles found in MS. Flow cytometric analysis of both resting and activated human platelets showed binding of the L4 mAb specific for the extracellular domain of the P2X7 receptor. Platelet-derived microparticles were also shown to express P2X7 receptors. Western analysis of platelet lysates from both MS patients and normal controls showed the presence of a 65-70kD protein staining with a sheep polyclonal antibody against residues 64-81 of human P2X7. The function of platelet P2X7 was studied in gel-filtered platelets loaded with the Ca-sensitive dye Fluo-4, and suspended in K-Tyrode's medium with 0.5mM Ca2+. Addition of ATP (0.5-5mM) failed to increase platelet Ca2+ while thrombin (1U/ml) gave a rapid Ca2+ increase. In conclusion, platelets express several purinergic receptors (P2X1, P2Y1, P2Y12), but the presence of P2X7 has not been previously reported. However, the function of P2X7 in platelets remains to be established.