Streptokinase from haemolytic bacteria, Streptococcus sp., has been known to act as an activator for the conversion of human plasminogen into the catalytically active plasmin (E.C. 188.8.131.52). However, streptokinase itself was found to be inactive catalytically. Rather its complex form with human plasminogen seems to be have a specific hydrolytic activity toward human plasminogen. Using the synthetic substrate such as N-$\alpha$-Cbz-L-lysine p-nitrophenyl ester, the activity of streptokinase-human plasminogen complex was determined. A series of experiment were carried out to find out the condition that the streptokinasehuman plasminogen complex free from plasmin activity was formed. From the results of experiments it was found that the added streptokinase was fully converted into its active complex in the condition of excess plasminogen with regard to streptokinase, and that so formed complex converted plasminogen to plasmin. Also it was observed that the activity of plasmin so formed was selectively inhibited by soybean trypsin inhibitor. As a result it was recognized that this complex was stoichiometrically formed according to amounts of streptokinase and plasminogen. The specificity of streptokinase-plasminogen complex for synthetic esters and macromolecules was investigated to obtain a information of active site of active complex. The results indicate that both positively charged and hydrophobic groups in substrate are required for binding capability to the complex. Also it was shown that the complex had a specificity for macromolecules. In addition it was observed that the activity of complex was increased in the presence of human fibrinogen which took a part in blood coagulation mechanism. On the basis of chemical modification study it is suggested that histidine and carboxylic groups are involved in the active site of streptokinase-plasminogen complex.