Helicobacter pylori antigen was prepared from an isolate from a patient with a duodenal ulcer. Serum sampleswere obtained from culture-positive H. pylori infected patients with duodenal ulcers, gastric ulcers and gastritis(n=30). As controls, three kinds of sera without detectable H. pylori IgG antibodies were used: 30 from healthyindividuals without history of gastric disorders, 30 from patients who were seen in the endoscopy clinic but wereH. pylori culture negative and 30 from people with other diseases. OFF-GEL electrophoresis, SDS-PAGE andWestern blots of individual serum samples were used to identify protein bands with good sensitivity and specificitywhen probed with the above sera and HRP-conjugated anti-human IgG. Four H. pylori protein bands showedgood (≥ 70%) sensitivity and high specificity (98-100%) towards anti-Helicobacter IgG antibody in culturepositivepatients sera and control sera, respectively. The identities of the antigenic proteins were elucidated bymass spectrometry. The relative molecular weights and the identities of the proteins, based on MALDI TOF/TOF, were as follows: CagI (25 kDa), urease G accessory protein (25 kDa), UreB (63 kDa) and proline/pyrroline-5-carboxylate dehydrogenase (118 KDa). These identified proteins, singly and/or in combinations, may be usefulfor diagnosis of H. pylori infection in patients.