We have evaluated the effect of lysine binding sites in kringle structures on the activation of plasminogen with plasmin and staphylokinase (SAK) complex and on the binding of plasminogen to SAK. Activation of native plasminogen (Glu-plasminogen) by a catalytic amount of plasmin-SAK complex increased in the presence of epsilon-amino-n-caproic acid (EACA) and then decreased with higher concentrations of EACA. By contrast, activation of modified plasminogen (Lys-plasminogen) decreased in an EACA-concentration-dependent manner. This decrease was explained by a more than 10-fold higher Km for activation of Lys-plasminogen with a catalytic amount of plasmin-SAK complex in the presence of EACA. EACA was a competitive inhibitor with Ki 0.23 mM. In addition, the Km for activation of mini-plasminogen, which lacks first four kringle structures (K1+2+3+4), was at least 3.5-fold higher than that for the activation of Lys-plasminogen. Furthermore, EACA showed a negligible inhibitory effect on the activation of mini-plasminogen by the plasmin-SAK complex. We observed a similar biphasic effect of EACA on the binding of Glu-plasminogen to SAK and a dose-dependent effect on the Lys-plasminogen binding to SAK by gel filtration methods. Since EACA binds to plasminogen via lysine binding sites in the kringle structure, we propose that the lysine binding site in K1+2+3+4 domain plays a role in the activation of plasminogen by plasmin SAK complex, and in the binding of plasminogen to SAK.