The defibrinogenating effect of batroxobin (Defibrase) in male Wistar rats and the inhibitory effects of the plasma of batroxobin-treated rats on the migration of human vascular smooth muscle cells (SMCs) were investigated in vitro. At 1 h after a single intravenous injection of 3.0, 10.0 or 30.0 BU/kg batroxobin (ten rats in each group), the fibrinogen levels in the plasma of the rats decreased to 88.3, 66.2 and 16.5%, respectively, of that in the plasma of control saline-treated rats (261.0+/-26.7 mg/dl). When the plasma from the batroxobin-treated rats was added to Dulbecco's modified Eagle's medium at a concentration of 0.2% for a vascular SMC migration assay and incubated in a modified Boyden's chamber system at 37 degrees C for 24 h, significant inhibitory effects on vascular SMC migration were observed in the 10.0 (P<0.05) and 30.0 BU/kg (P<0.01) batroxobin-treated rats. The plasma of batroxobin-treated rats as well as standard rat fibrinogen induced vascular SMC migration in a fibrinogen content-dependent manner except the plasma of the 30.0 BU/kg batroxobin-treated rats. Moreover, the rat serum (0.1 approximately 5.0%) did not show any activity on vascular SMC migration in the present experimental system. These results indicate that the plasma fibrinogen significantly influences vascular SMC migration, and that the inhibitory effect of the plasma of batroxobin-treated rats on vascular SMC migration is related to the defibrinogenating action of batroxobin in vivo.