Background: Phosphatidylserine-expressing microparticles circulate in blood with a short half-life of 10 minutes. We tested the role of an endothelium-derived phosphatidylserine-binding opsonin, developmental endothelial locus-1 (Del-1), in the uptake of platelet microparticles. Methods and Results-Cultured human umbilical vein and microvascular endothelial cells avidly engulf BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene)-maleimide- labeled platelet microparticles. Microparticle uptake was inhibited by a monoclonal antibody to Del-1 (P=0.027) and by annexin A5 (P=0.027), abciximab (P=0.027), a monoclonal antibody to integrin αVβ3 (P=0.027), and chlorpromazine (P=0.027). These results suggest that Del-1 mediates phosphatidylserine- and integrin-dependent endothelial uptake of microparticles by endocytosis. To assess the in vivo significance, we infused fluorescent platelet microparticles into the inferior vena cava of mice and harvested endothelial cells from the pulmonary and systemic circulation. Compared with their wild-type littermates, Del-1- deficient mice had decreased uptake in endothelial cells in lung (3.07±1.9 versus 1.09±1.3, P=0.02) and liver (2.85±1.1 versus 1.35±0.92, P=0.01). Furthermore, after endotoxin administration, Del-1- deficient mice displayed an increase in the level of microparticles compared with wild-type mice (P=0.02). Conclusions-These studies show a physiological role for Del-1 in the clearance of phosphatidylserine-expressing microparticles by endothelium. (Circulation. 2012;125:1664-1672.)