The C-type lectin-like receptor CLEC-2 mediates platelet activation through a hemimmunoreceptor tyrosine-based activation motif (hemITAM). CLEC-2 initiates a Srcand Syk-dependent signaling cascade that is closely related to that of the 2 platelet ITAMreceptors: glycoprotein (GP)VI and Fc?RIIa.Activation of either of the ITAMreceptors induces shedding of GPVI and proteolysis of the ITAM domain in Fc?RIIa. In the present study, we generated monoclonal antibodies against human CLEC-2 and used these to measureCLEC-2expressiononrestingandstimulatedplateletsandonotherhematopoietic cells.We showthatCLEC-2 is restricted to platelets with an average copy number of 2000 per cell and that activation of CLEC-2 induces proteolytic cleavage ofGPVI and Fc?RIIa but notof itself.WefurthershowthatCLEC-2andGPVI are expressedonCD41+microparticlesinmegakaryocyteculturesandinplatelet-rich plasma, which are predominantly derived from megakaryocytes in healthy donors, whereas microparticles derived from activated platelets only express CLEC-2. Patients with rheumatoid arthritis, an inflammatory disease associated with increased microparticle production, had raised plasma levels of microparticles that expressed CLEC-2 but not GPVI. Thus, CLEC-2, unlike platelet ITAM receptors, is not regulated by proteolysis and can be used to monitor platelet-derived microparticles.