The functions of Helicobacter pylori urease, which causes chronic gastritis and gastric ulcers, were investigated in detail at the molecular level. Twenty-six monoclonal antibodies (HpU-1 to HpU-26) were produced against H, pylori urease, which was highly purified in two purification steps. The monoclonal antibodies obtained were characterized with respect to their specificity, immunoaffinity, and inhibitory effect on the enzymatic activity of the urease. The affinity constants of the monoclonal antibodies to the urease were in the range from 1 x 10(6) to 5 x 10(9)/M, Cross-reactivity experiments with other proteins including Jack bean urease, indicated that these affinities were highly specific to the urease. The antibodies produced could be mainly divided into two categories on the basis of their recognition of the antigen＇s subunits : one group recognized the alpha-subunit of the urease and the other recognized the beta-subunit. Among the 26 monoclonal antibodies obtained, the antibody designated as HpU-2 showed the strongest inhibitory effect on the enzymatic activity of the urease suppressing the urease activity by 82%. HpU-2 recognized the alpha-subunit of the urease, but not beta-subunit which is considered to include the active site of the urease for binding the nickel ion to hydrolyze urea to ammonia. Its apparent sanity constant was 7.5 x 10(8)/M, The inhibitory effect was enhanced by making two-antibody cocktails of HpU-2 with various other monoclonal antibodies.