AIM: To study the local hemorrhagic activity of a fibrinolytic enzyme (FIIa) from Agkistrodon acutus venom and its mechanism. METHODS: The local hemorrhagic activity was determined by subcutaneous injection on the back of mouse. The effects of FIIa on factor X, prothrombin, gelatin, and collagen were visualized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Platelet aggregation assays were performed in rat plateletrich plasma (PRP). Human umbilical vein endothelial cells (HUVEC) were cultured and passaged in complete M199 medium. Cell viability and nuclear morphology change were determined by fluorescein diacetate (FDA) staining and Hoechst 33258 staining, respectively. RESULTS: The minimum hemorrhagic dose (MHD) of FIIa was 89 μg.In vitro, FIIa (0.25 g/L) degraded factor X, prothrombin, collagen, and gelatin, and dose-dependently (0.25, 0.50,0.75, and 1.00 g/L) inhibited the platelet aggregation induced by ADP in rat PRP. When HUVEC in culture treated with FIIa, HUVEC showed detachment in a dose-dependent manner, but no apoptosis sign was observed.CONCLUSION: FIIa had local hemorrhagic activity, and the mechanism was related to the degradation of factor X,prothrombin, gelatin, and collagen, the inhibition of ADP-induced platelet aggregation, and inducement of HUVEC detachment.