A proteolytic enzyme with high activity on casein was purified from Lachesis muta snake venom. This protein called "Proteinase I" was obtained using a gel filtration chromatography on Sephadex G-100 at pH 6.5, 0.1 M Ammonium acetate buffer, followed by ion exchange chromatography on DEAE-Cellulose at pH 7.5 and re-chromatographed on DEAE-Cellulose at pH 9.0 and 7.8 in Tris-HC1 buffer. A homogeneous band was obtained with the isolated protein on polyacrylamide gel electrophoresis. A molecular weight of 25,100 by gel filtration and an optimum pH of 8.4 were found for this enzyme. A total enzymatic activity was kept after a heating at 45 degrees C for ten minutes while the activity at 70 degrees C was 4% only. Synthetic esters as TAME and BAEE were not attacked by this enzyme. The activity was not affected by calcium ions and hemorrhagic action was not observed either.