Abstract-Abnormal fibrin architecture is thought to be a determinant factor of hypofibrinolysis. However, because of the lack of structural knowledge of the process of fibrin digestion, relationships between fibrin architecture and hypofibrinolysis remain controversial. To elucidate further structural and dynamic changes occurring during fibrinolysis, cross-linked plasma fibrin was labeled with colloidal gold particles, and fibrinolysis was followed by confocal microscopy. Morphological changes were characterized at fibrin network and fiber levels. The observation of a progressive disaggregation of the fibrin fibers emphasizes that fibrinolysis proceeds by transverse cutting rather than by progressive cleavage uniformly around the fiber. Plasma fibrin clots with a tight fibrin conformation made of thin fibers were dissolved at a slower rate than those with a loose fibrin conformation made of thicker (coarse) fibers, although the overall fibrin content remained constant. Unexpectedly, thin fibers were cleaved at a faster rate than thick ones. A dynamic study of FITC-recombinant tissue plasminogen activator distribution within the fibrin matrix during the course of fibrinolysis showed that the binding front was broader in coarse fibrin clots and moved more rapidly than that of fine plasma fibrin clots. These dynamic and structural approaches to fibrin digestion at the network and the fiber levels reveal aspects of the physical process of clot lysis. Furthermore, these results provide a clear explanation for the hypofibrinolysis related to a defective fibrin architecture as described in venous thromboembolism and in premature coronary artery disease. Key Words: fibrin Ⅲ fibrinolysis Ⅲ confocal microscopy T he fibrin matrix has a much more complicated role than that of providing the scaffolding of the thrombus or being the target of fibrinolysis. Abnormal fibrin structure in vitro has been related to in vivo premature coronary artery disease in young patients and to severe venous thromboembolic disease in patients with dysfibrinogenemias. 1,2 In those situations, an abnormal fibrin matrix made up of abnormally thin fibers has been shown to promote hypofibrinolysis and embolization. Although much is known about the molecular basis of fibrinolysis, relationships between fibrin conformation and fibrinolysis need to be clarified.Fibrin actively regulates its self-dissolution through numerous interactions with fibrinolytic and antifibrinolytic components. Activation of plasminogen by tissue plasminogen activator (tPA) that is initiated on the conversion of fibrinogen into fibrin is a critical step that is affected by fibrin structure. The theory of a decrease of plasminogen binding to fibrin 3 has been strengthened from observations showing that clots with a fine fibrin (tight) conformation display a slower lysis rate than those with a coarse fibrin (loose) conformation. 2,4,5 So far, neither a molecular nor a structural basis has been detected for these differences. Moreover, a recent report demonstrates that under othe...