We examined the role of coagulation-fibrinolysis system in Theiler's murine encephalomyelitis virus-induced demyelinating disease (TMEV-IDD). The degree of fibrin deposition around the vessels in the spinal cord was significantly higher in susceptible SJL/J mice on 30 days post intracerebral injection (i.c.) than resistant C57BL/6 mice on 30 days post i.c. or mock infected SJL/J mice. Treatment with batroxobin (30 BU/kg/day), which is a thrombin-like defibrinogenating enzyme, causing a profound degree of afibrinogenemia, suppressed clinical signs of TMEV-IDD. Plasma fibrinogen concentration was significantly decreased in batroxobin-treated mice. Histologically, though the degree of perivascular mononuclear cell infiltration in the spinal cord was not suppressed in batroxobin-treated mice compared to saline-treated control mice, fibrin deposition was markedly suppressed in batroxobin-treated mice. These findings suggest that batroxobin suppresses TMEV-IDD through its defibrination effect, and provide evidence that CNS-associated deposition of fibrin and ensuing fibrinolysis, together with increased permeability of the blood-brain barrier (BBB), are prerequisite events for clinical manifestations of TMEV-IDD.