AbstractCultured bovine aortic endothelial cells have a relatively complex flbrinolytic system that is responsive to both the physiological state of the cell itself and to a variety of agents added to the culture medium. The flbrinolytic activity of these cells results from the production of both urokinase‐type and tissue‐type plasminogen activators and is regulated by an inhibitor capable of neutralizing their activities. The properties of these flbrinolytic components will be reviewed, and their respective roles in initiating and regulating the flbrinolytic activity of the cells will be summarized. A cDNA coding for the inhibitor has been isolated, and its sequence will be compared to that of other serine proteinase inhibitors.