Human endothelial cells cultivated on polystyrene microcarrier beads were used to study endothelial anticoagulant activity in vitro. Spontaneous whole blood coagulation was inhibited by endothelial cells on microcarriers at a surface to volume ratio of 16 cm2/ml blood. Thrombin activity generated in nonanticoagulated whole blood during 1 hour and assessed by its fibrinogen clotting effect was reduced by 87% in the presence of endothelial cells. Consistent with this observation, prothrombin fragment1+2, fibrinopeptide A, and thrombin-antithrombin III-complex release during the same period of time were inhibited by 81%, 47%, and 88%, respectively. Immunoblotting analysis of cell-free supernatants derived from the same samples demonstrated that prothrombin activation was strongly suppressed in the presence of endothelial cells. Furthermore, the incubation of nonanticoagulated whole blood with endothelialized beads for only 5 minutes after venipuncture was sufficient to prevent subsequent prothrombin activation in the cell-free supernatants of the same whole blood sample after centrifugation. These findings suggest that interruption of the coagulation cascade is probably one major mechanism of endothelial anticoagulant activity in vivo.