In the present study we investigate the fibrin(ogen)-endothelial cell binding and the effect of thrombin on the endothelial cells in relation to fibrin(ogen) binding capacity. Endothelial cell fibrinogen binding was concentration and time-dependent, reaching saturation at 1.4 μM of added ligand. At equilibrium, the number of fibrinogen molecules bound per endothelial cell in the monolayer was 5.8±0.7×10(6). When endothelial cells were activated by different concentrations of thrombin (0-0.1 NIH units ml(-1)), no increase in fibrinogen binding capacity was observed at all the thrombin concentration tested. Whereas disruption of endothelial cell monolayers was observed at thrombin concentrations higher than 0.05 NIH units ml(-1), no increase in the amount of fibrinogen bound was observed. Therefore, resting and thrombin-activated endothelial cells show the same fibrinogen binding capacity.The adhesion of endothelial cells in suspension on immobilized fibrinogen or fibrin was studied to ascertain whether the behavior of fibrin is similar to that of fibrinogen. The extent of endothelial cell attachment to immobilized fibrinogen and fibrin was similar (4275±130 cells cm(-2) for fibrinogen and 4350±235 cells cm(-2) for fibrin) and represent approximately 40% of the added endothelial cells. However, endothelial cell adhesion to immobilized fibrin was significantly faster than endothelial cell adhesion to immobilized fibrinogen. The maximum binding rate was 66±9 and 46±8 cells cm(-2) min(-1) for fibrin and fibrinogen, respectively. Therefore, the fibrinopeptides released by thrombin from fibrinogen induce qualitative changes which enhance the fibrin interaction with the endothelial cells.