The plasma zymogen, plasminogen, and its activated product, plasmin, are major components of the human fibrinolytic system, which also includes: the fibrin clot and its precursor protein, fibrinogen; activators of plasminogen; and inhibitors of both plasmin and plasminogen activators. Intrinsic plasminogen activators are those that are found in plasma. A latent plasmin light chain is also a component of the plasminogen structure. The activation mechanism of human plasminogen by streptokinase, and likely staphylokinase, proceeds along different lines. Activation of plasminogen by urokinase is also somewhat enhanced by the presence of fibrinogen, fibrin, and their degradation products. The activation of human plasminogen by streptokinase has been shown to be regulated by both fibrinogen and anions. The amino acid sequence of human plasminogen is unusual in that five regions of approximately 78 to 80 amino acid residues each, all present in the amino terminal 560 amino acids, appear, visually, to be highly homologous to each other.