Because the influence of fibrin on the reaction of plasminogen activation by various plasminogen activators is different, the kinetic constant of the reaction of plasminogen activation catalyzed by InB with and without fibrin were detected. The result is: K_m^fibrin＝4.2 μmol·L^-1，greater than the normal K_m=0.379 μmol·L^-1; k_cat^fibrin＝0.107 s^-1，greater than the normal k_cat=0.0165 s^-1. The results suggest that existence of fibrin in the reaction system of plasminogen activation depress the affinity between InB and plasminogen, but accelerates the hydrolysis of plasminogen by InB. The count up effect is inhibition.