The steady-state kinetics of the amidolytic activity of single chain tissue-type plasminogen activator (tPA) were analyzed in the presence or absence of different molecular forms of fibrinogen degradation products. Single chain tPA showed a Km value of 1.6 mM and kcat value of 4.9/s toward the chromogenic substrate H-D-Ile-Pro-Arg-p-nitroanilide (S-2288). In the presence of infinite concentrations of fibrinogen, kinetic constant was calculated as about 8-times higher than that in the absence of fibrinogen, mainly caused by the decrease of Km value. The dissociation constant (Ka) for this stimulation by fibrinogen was 2.9 microM. When the same assay was conducted with fragment X or fragment D of fibrinogen, the kinetic constants increased 3.2 and 2.9-times, respectively, whereas no enhancement was obtained by fragment E. Neither lysine analogues nor monoclonal antibody toward domains of finger and epidermal growth factor of tPA quench the enhancement by fibrinogen. This enhancement was not observed in the case of the two chain form of tPA. These results indicate that fibrinogen enhances the amidolytic activity of single chain tPA by binding to kringle 2 domain or light chain through D domain of fibrinogen.