Human endothelial cells express antithrombotic properties by producing prostacyclin, heparan sulfate and plasminogen activator (PA). Using an established cell line, TKM-33, from human umbilical vein endothelial cells, the pericellular urokinase-type PA (u-PA) activity and expression of u-PA receptor (u-PAR) were investigated. The endothelial cells produced and secreted large amounts of u-PA and low levels of tissue-type PA (t-PA) and of PA inhibitor-1 (PAI-1), which were identified by immunohistochemical study and electrophoretic enzymography. Diisopropylfluoro-phosphate-treated ¹²⁵I-u-PA bound specifically to acid-treated monolayered endothelial cells with a K_d of 3.46±1.17 nM, and B_max of (0.09±0.04)×10⁶ sites/cell. mRNA of u-PAR was detected by using Northern blot analysis. Thus, these endothelial cells express u-PAR which bounds u-PA specifically. Phorbol myristate acetate (PMA) stimulation to the endothelial cells altered the K_d value to 3.18 ± 0.64 nM, and B_max value to (0.19±0.10)×10⁶ sites/cell, respectively. PMA treatment of endothelial cells increased u-PAR mRNA. Similarly, H7-treated endothelial cells showed a dose-dependent increase of u-PAR mRNA. However, PMA and H7 did not stimulate the expression of u-PA and t-PA mRNAs significantly. The expression of PAI-1 mRNA was increased by both PMA and H7. These findings suggest that the established endothelial cell line, TKM-33, possesses the character of endothelial cells and expresses u-PAR on their cell surface which is occupied by intrinsic u-PA secreted from the cells. The pericellular u-PA activity and the expression of u-PAR were regulated by protein kinase pathway.