FIa, a factor X activator, was isolated from the venom of Daboia russellii siamensis (Myanmar) after a series of chromatographic separations. FIa displayed procoagulant activity by shortening plasma recalcification time and converted human factor X (FX) to activated human factor X (FXa) by cleaving the heavy FX chain, possibly at the Arg51-Ile52 peptide. FIa was positive in a glycoprotein staining test, demonstrating that it is a glycoprotein. Optimal temperature and pH values were important for FIa procoagulant activity. Procoagulant activity was maintained above 85% of the initial activity at pH 7.0~8.0, and showed equally maximum activity at temperatures ranging from 30 to $50^{circ}C$. In addition, FIa procoagulant activity was completely inhibited by EDTA (5 mM), but not by PMSF (10 mM), suggesting that it is a metalloproteinase.