The binding of iodine-labelled plasminogen to Helicobacter pylori CCUG 17874 was characterized. Inhibition of the binding was observed after preincubation of H. pylori cells with nonradiolabelled plasminogen, lysine, or the lysine analogue -aminocaproic acid. Fragments of plasminogen, kringles 1 to 3, kringle 4, and mini-plasminogen, were also studied as potential inhibitors. Mini-plasminogen caused total inhibition of the plasminogen binding, while the other fragments caused only partial inhibition. These findings suggest that H. pylori binds specifically the fifth kringle structure of the plasminogen molecule. Plasminogen binding to H. pylori seems to be independent of culture media and independent of the presence of the cytotoxin-associated CagA antigen. Immunoblot analysis identified two plasminogen binding proteins of 57 and 42 kDa. Scatchard plot analysis revealed one binding mechanism with a K_d value of 7 × 10⁷ M. Conversion of H. pylori cell-bound plasminogen to plasmin in the presence of a tissue-type plasminogen activator was demonstrated by digestion of the chromogenic substrate S-2251. No activation was noted when plasminogen or tissue-type plasminogen activator was incubated with H. pylori cells alone. Formation of H. pylori cell surface-bound plasmin may be important to provide a powerful proteolytic mechanism for gastric tissue penetration in type B gastritis and peptic ulcer disease, since plasmin degrades not only fibrin but also extracellular matrix proteins such as various collagens and fibronectin.