The activation of plasminogen to an active fibrinolytic enzyme, plasmin, by Streptokinase and Staphylokinase was studied using caseinolytic assay method. The Staphylokinase producing microorganism was selected among 15 $\beta$-hemolytic Staphylococci and that of Streptokinase was purchased from ATCC. Staphylokinase was purified by CM-cellulose ion exchange chromatography and Seph 4B-EACA column chromatography. Ammonium sulfate fractionation and DEAE-cellulose ion exchange chromatography were used to purify Streptokinase. Both of them showed a very limitted substrate specificity toward plasminogen, and their catalytic activities were inhibited by the presence of macromolecular proteins such bovine serum albumin and casein. Both activators showed a similar mode of catalytic action on plasminogen, but their nature and molecular properties were found to be different from that of human urokinase. Based on kinetic experiments and other inhibition study, the reaction mode of these two activators was discussed. Also, by chemical modification, the ionic interaction between Streptokinase and Plasminogen to form complex was stuied.