When Glu-plasminogen (Glu-plg) was activated by various amounts of streptokinase (SK), Km of the hydrolysis of S-2251 by the mixture of Glu-plg and SK did not change, but Vmax increased with an increase in the amount of SK. Since low concentrations of SK-plg complex do not result in its conversion to the SK-plasmin complex by mutual activation, these results seem to suggest that the SK-plg complex may be a better activator when S-2251 is used as a substrate. When Glu-plg was activated by SK in the presence of fibrin(ogen) or its degradation products (potentiating agents), Km did not change with an increase in the concentration of each potentiating agent, but Vmax increased. The effects of potentiating agents on the kcat E.