T he endothelial cell protein C receptor (EPCR) binds protein C and facilitates activation by the thrombin-thrombomodulin complex. EPCR also binds activated protein C (APC) and inhibits APC anticoagulant activity. In this study, we detected a soluble form of EPCR in normal human plasma. Plasma EPCR appears to be approximately 43, 000 D, and circulates at approximately 100 ng/ml (98.4+/-27.8 ng/ml, n = 22). Plasma EPCR was purified from human citrated plasma using ion exchange, immunoaffinity, and protein C affinity chromatography. Flow cytometry experiments demonstrated that plasma EPCR bound APC with an affinity similar to that previously determined for recombinant soluble EPCR (Kdapp = 30 nM). Furthermore, plasma EPCR inhibited both protein C activation on an endothelial cell line and APC anticoagulant activity in a one-stage Factor Xa clotting assay. The physiological function of plasma EPCR is uncertain, but if the local concentrations are sufficiently high, particularly in disease states, the present data suggest that the soluble plasma EPCR could attenuate the membrane-bound EPCR augmentation of protein C activation and the anticoagulant function of APC.