Human glomerular epithelial cells (GECs) in culture synthesize single-chain, urokinase-type plasminogen activator (SC-uPA), tissue-type plasminogen activator (t-PA), and plasminogen activator inhibitor 1 (PAI-1) and possess specific membrane-binding sites for U-PA. Using purified '251-alpha thrombin, we demonstrate here the presence of two populations of specific binding sites for thrombin on GECs (1.Kd = 4.3 5 1.0 x lo-'' M, 5.4 2 1.4 x l o 4 M sites per cell, 2. Kd = 1.6 5 0.5 X lop8 M, 7.9 2 1.8 x 10' sites per cell). Purified human alpha thrombin promoted the proliferation of GECs and induced a timeand dose-dependent increase of SC-uPA, t-PA, and PAL1 antigens released by GECs. Thrombin-mediated increase in antigen was paralleled by an increase in the levels of corresponding U-PA and PAL1 messenger RNA. In contrast, thrombin decreased u-PA activity in conditioned medium. This discrepancy between u-PA antigen and U-PA activity was explained by a limited proteolysis of SC-uPA by thrombin, leading to a two-chain form detected by immunoblotting and that could not be activated by plasmin. Thrombin also decreased the number of U-PA binding sites on GECs (p < 0.05) without changing receptor affinity. Hirudin inhibited the binding and the cellular effects of thrombin, whereas thrombin inactivated by diisopropylfluorophosphate had no effect, indicating that both membrane binding and catalytic activity of thrombin were required. We conclude that thrombin, through specific membrane receptors, stimulates proliferation of GECs and decreases the fibrinolytic activity of GECs both at the cell surface and in the conditioned medium. These results suggest that thrombin could be involved in the pathogenesis of extracapillary proliferation and persistency of fibrin deposits in crescentic glomerulonephritis.