Two characteristic monoclonal antibodies (HpU-2 and -18) out of 26 monoclonal antibodies (HpU-1 similar to26) produced against Helicobacter pylori (H. pylori) urease showed a strong inhibitory effect against the enzymatic activity of the urease. Epitope mapping about some monoclonal antibodies of the HpU-series inhibiting enzymatic activity was performed by using a surface plasmon resonance apparatus and by digesting H. pylori urease with trypsin, followed by mass spectroscopy. The sequences of the epitopes recognized by HpU-2 and -18 were SVELIDIGGNRRIFGFNALVDR (22 mer) and IFGFNALVDR (10 mer), respectively. The former sequence is present as a part of a loop structure at a position close to the C-terminal of the a-subunit of H. pylori urease, although it has been suggested that the active site of the urease resides in the betasubunit. The above peptide (22 mer) was chemically synthesized in a linear and cyclic form, and its conjugate with BSA was immunized in rabbits. The resultant serum induced by the linear form could specifically bind to H.pylori infecting human gastric mucosa. These results suggest that the above sequence (22 mer) must be an important epitope, although it locates in the (alpha-subunit but not in the beta-subunit. (C) 2004 Wiley Periodicals, Inc.