Plasminogen activator inhibitor 1, a serpin, binds to fibrin-associated tissue plasminogen activator. The resulting stable complex remains associated with fibrin but cannot activate plasminogen (Wagner et al. 1989). The importance of this step in the regulation of clot dissolution in vivo is indicated by the occurence of thrombosis in individuals with abnormally little tissue plasminogen activator or abnormally much plasminogen activator inhibitor (Juhan-Vague et al. 1987).