Investigation of a new clotting factor for use as molecular model in thrombosis. Crude lyophilized Bothrops pirajai snake  venom was initially gel  filtrated on a Sephadex G-75 columm equilibrated and eluted with 0.05 M,  ammonium bicarbonate buffer, pH 8.0. The clotting fraction, on human fibrinogen, was rechromatographed by  High performance liquid chromatography (HPLC) on an ion exchange column of ES 502 N (Asahipak). The isolated active protein was assayed for purity by page under reducing conditions and then chemically (carbohydrate content, amino acid composition, Mr, pI and N-terrminal sequencing) and biologically (clotting, fibrinolytic, fibrinogenolytic and platelet aggregating activity) characterized. The enzyme, named Piraserbin, showed Mr 37,500, 12% neutral carbohydrates and pI 4.6, characterized as an acidic monomeric glycoprotein with N-terminal sequence VLGGDDCNIN. Its minimum coagulant dose was 1.75 mg, acting preferentially on the a chain of fibrinogen. It also aggregates 60% platelets from platelet rich plasma (PRP), only 1.2 times less than ADP. Fibrinolytic activity was detected only at high amounts of enzyme (100 mg), with an especific activity of 50 U/mg. Piraserbin is a new clotting factor isolated from snake venoms and, similarly to other clotting enzymes, represents an important tool for structure function studies and use in the medical field.