Abstract: Snake venoms display a variety of proteins and peptides able to affect thrombosis and hemostasis, including serinoproteases which constitute 20% of the total proteins of the venom.A serinoprotease from B. pirajai snake venom was isolated and biochemically characterized.It was named piraserbin.Its high purity level was shown by SDS polyacrylamide gel electrophoreses under reducing conditions, showing a single chain with Mr 37,500, 12% neutral carbohydrate and pI 4.6.Its minimum coagulant dose was 1.75m g and its fibrinogenolytic activity was mainly upon the a chain of fibrinogen, and its fibrinolytic activity was observed only at high enzyme dose (100m g).It showed a high esterase activity upon TAME (15U/mg) and high amidolytic activity upon synthetic peptides S2302 ( H-D-Pro-Phe-Arg-pNA-2 HCl) which is a substrate for plasma calicrein, and S 2238 ( H-D-Phe-Pip-Arg-pNA-2HCl), substrate for thrombin.When assayed for cinin release, it did not show any activity.It was however able to induce platelet aggregation of 90% at 20m g/ml.N-terminal sequencing reveated 90% homology with crotalase (calicrein-like serinoprotease from Crotalus adamanteus venom).