Background: Microparticles may be generated from a number of cell types and are known to play a role in haemostasis by a variety of mechanisms. We investigated the role of platelet, red cell, and leucocyte-derived microparticles in the measurement of thrombin generation.Methods: Four parameters of thrombin generation (the endogenous thrombin potential (ETP), lag time, time to peak, peak height) and microparticle content was determined in 35 plasma samples from normal individuals pre and post filtration to remove microparticles. Immunofluorescent flow cytometry was used to identify and enumerate platelet, leucocyte, monocyte and red cell derived microparticles in plasma samples based on the expression of CD42b, CD45, CD15, and Glycophorin A respectively. Expression of phosphatidylserine and tissue factor by microparticles was determined by Annexin V and anti CD142 binding. The pre and post filtration results were compared.Results: There was a significant decrease in ETP and Peak Height, and an increase in the time to peak post filtration (P < 0.001). A significant decrease in the number of CD421, CD451, CD151, CD1421, and Annexin V1 microparticles was also observed. The change in CD42b1 microparticles correlated highly with the change in Annexin V1 microparticles (r 5 0.68). Whilst the change in ETP correlated best with the change in CD151 microparticles (r 5 0.45) and the change in time to peak correlated with the change in Annexin V binding (r 5 0.52) (P < 0.01).Conclusion: The presence of micropartcles in plasma significantly affects thrombin generation. V C 2010