Production of tissue factor (TF) in response to lipopolysaccharide (LPS) was examined in human umbilical vein endothelial cells (HUVECs) transfected with human CD14 DNA. The expression of CD14 on HUVECs dramatically enhanced the production of TF at a low concentration of LPS in the absence of fetal calf serum (FCS). On the other hand, mock-transfected HUVECs did not respond to even a high concentration of LPS. TF production in CD14-expressing HUVECs was significantly inhibited by anti-CD14 monoclonal antibody. Addition of FCS to the culture of CD14-expressing HUVECs markedly augmented the LPS-induced TF production, whereas only a marginal effect was observed in mock-transfected HUVECs. The findings suggested that the integration of membrane CD14 rendered HUVECs highly sensitive to LPS in the production of TF irrespective of the presence of FCS.