From the snake venom of Agkistrodon acutus, two proteases, acuthrombin-A and acuthrombin-C, were isolated and purified to homogeneity. They can cleave the human fibrinogen to release the fibrinopeptide A and fibrinopeptide B with specific activity of 120 and 370 NIH units/mg, respectively; the fibrinogen-clotting activity can be inhibited distinctly by PMSF or DFP or EDTA, but not by heparin. The two proteases show also arginine-esterase activity hydrolyzing some synthetic substrates such as TAME and BAEE. Additionally, they are glycoproteins with an average content of 2.4% (acuthrombin-A) and 2.1% (acuthrombin-C) neutral carbohydrates, respectively. Acuthrombin-A has a MW of 13,900 as estimated by SDS-PAGE under reduced or nonreduced conditions and 28,000 as determined by gel filtration. For acuthrombin-C, there were two protein bands corresponding to MW of 13,900 and 14,800 on SDS-PAGE with different darkness under reduced or nonreduced conditions, while its MW was estimated to be 69,000 by gel filtration. The isoelectric points were 7.5 for acuthrombin-A and 5.0 for acuthrombin-C by isoelectric focusing. Neither acuthrombin-A nor acuthrombin-C has haemorrhagic or lethal activity. Acuthrombin-A has also a small amount of activity to activate the Factor XIII.