Abstract:A protease was crystallized from the fresh leaves of Agave sisalana by means of ethanol fractionation and chromatography on DEAE-cellulose. The enzyme crystals seen under microscope were shaped as regular hexogonal plants. The enzyme could be activated by cysteine and EDTA, and was reversibly inhibited by p-chlor omercuribenzoate, Hg2+, Ag+, Cu2+ and 5,5‘-dithiobis (2-nitrobenzoic acid), irreversibly inhibited by iodoacetic acid at pH 7.5. These resuits showed that sulphydryl group was essential to the enzyme activity. The optimal pH of the enzyme for casein was about 7 5, and the optimal temperature was about 50℃. The Km value for casein was 0.0625 %. It was relatively stable at temperature below 45℃ and at the. pH ranging from 6.0 to 10.0.