The activation of human plasminogen to active fibrinolytic enzyme plasmin (EC 126.96.36.199) was studied with streptokinase. The activation of human plasminogen to an active enzyme plasmin is occured via hydrolytic cleavage of Arginyl-Valyl endopeptide bond. Several forms of the plasminogen activator have been found many different source including humoral origin. Among them, an urinary activator, urokinase, has been studied mostly and thus used as a prototype activator for the biochemical mechanism of the plasminogen activation process. However, a series of study on the plasminogen activation by bacterial activator, streptokinase, from $\beta$-haemolytical pathogenic bacteria, indicates that its mode of action is is quite different from that by urokinase. In this communication, we report our findings on the nature of streptokinase as an allosteric protein requiring human plasminogen for the catalytic activity. When the amount of streptokinase is fixed and the plasmingen concentration is varied, the concentration dependent rate profile showed a typical saturation curve. However, when the ratio of plasminogen and streptokinase was small, the rate profile becomes sigmoidal. The kinetic data along with other results suggest that streptokinase forms a complex with plasminogen to become an active enzyme, but once bound with plasminogen it becomes an active enzyme.