Aim: To investigate the fibrin(ogen)olytic character of FⅡa isolated from Agkistrodon acutus venom in vitro and in vivo. Methods: 125I-labeled human plasma clot lysis was measured in vitro and rabbit carotid artery thrombosis was as an in vivo model. Results: In vitro, urokinase (UK) at 25, 35, 40, 45, 60 kU/L and FⅡa at 0.08, 0.23, 0.4, 0.5, and 0.7 g/L resulted an equivalent clot lysis (20%, 40%,50%, 60%, and 80%). UK at 25-60 kU/L induced 27.3%±3.6%, 35.2%±2.3%,39.3%±2.4%, 44.2%±4.6%, and 51.1%± 1.2% fibrinogen degradation. But FⅡa at 0.08-0.7 g/L induced 95.4%±0.3%, ＞95.6%, ＞95.6%, ＞95.6%, ＞95.6% fibrinogen degradation respectively. In vivo, UK 40 kU/kg and FⅡa 1.0 mg/kg reduced the weight of residual thrombus to 9.0±2.5 mg and 7.8±3.5 mg compared with negative control group (30.0±5.4 mg). But the fibrinogen degradation rate after UK 40 kU/kg and FⅡa 1.0 mg/kg treatment was 24.4%±6.2% and 4.1%±7.8%, respectively (P＜0.05, n=6). The order of the lysis speed after UK 125 kU/L treatment was platelet poor plasma (PPP) clots＞the whole blood clots＞platelet rich plasma (PRP) clots.The sequence for FⅡa 0.4 g/L was PRP＞PPP＞whole blood clots. Conclusion: At the same percentage of clot lysis, FⅡa degraded more fibrinogen than UK did in vitro but less fibrinogen than UK did in vivo. The order of the lysis speed was PPP＞whole blood clots＞PRP clots for UK and PRP＞PPP＞whole blood clots for FⅡa.