Large vessel and microvascular endothelial cells were compared in their capacity to synthesize and secrete coagulant and fibrinolytic factors. Human omental tissue microvascular endothelial cells (HOTMEC) and human umbilical vein endothelial cells (HUVEC) were isolated, grown to confluency under identical conditions, and studied in primary cultures. After an incubation period of 12 hours in serum-free medium, the conditioned medium of confluent HOTMEC contained 100-fold higher levels of tissue plasminogen activator (tPA) antigen than that of HUVEC. The conditioned media as well as the lysates of both cell types did not contain any free tPA activity, but the free plasminogen activator inhibitor capacity was found intracellularly as well as extracellularly. Although von Willebrand factor was detected in both cell types by immunofluorescence, measurable amounts were only found in HUVEC using an enzyme-linked immunosorbent assay. The kinetics of protein C activation by thrombin on the surface of once-passaged cells were identical for HOTMEC and HUVEC. The present study indicates that cultivated HOTMEC produce larger quantities of tPA than HUVEC do, possess smaller amounts of von Willebrand factor than HUVEC do, and express thrombomodulin for protein C activation as effectively as HUVEC.