The procoagulant from Notechis ater niger was purified by gel filtration and anion exchange chromatography. It is a protein with an approximate mol.wt of 58,000 and in the presence of B mercaptoethanol is reduced to two chains with mol.wts of 37,000 and 23,000. The procoagulant has a pI of 7.3. The whole venom requires factor V to be present to bring about coagulation while Ca2+ and phospholipid are not essential, but when present stimulate this process. Normal prothrombin, but not decarboxyprothrombin, is converted by the venom. The activity of the procoagulant from Notechis species has been equated with factor Xa and in this study the similarity is noted, while ecarin-like characteristics in not requiring Ca2+ and phospholipid and an ability to clot heparinised plasma were also noted.