Fraction G from Cerastes vipera venom previously purified on Sephadex G100 was refractionated on DEAE-Sephadex A50 column. A factor X activator was obtained. It had a mol. wt of 12,500 and an isoelectric point of 4.4. It shortened the plasma recalcification time of normal plasma, and plasmas deficient in factors V, VII, VIII, IX, XI and XIII, while it had no effect on plasma deficient in factor X or factor II. It had a serine protease activity and a minimal plasmin activity. PMSF, leupeptin and iodoacetamide exerted a pronounced inhibitory effect on its serine protease activity. Polyantivenin could neutralize the coagulant activity of the activator.