The distribution and localization of Helicobacter pylori (HP) and mononuclear cells (MNC) in the gastric mucosa were investigated immunohistochemically in 120 patients with gastritis or peptic ulcer. HP was detected in the gastric mucosa of 58% of 40 gastritis patients, 82% of 56 gastric ulcer (GU) patients, and 88% of 24 duodenal ulcer (DU) patients. In the HP-positive mucosa, cells positive for IgG or IgA were increased significantly when compared to the HP-negative mucosa. Neutrophils and eosinophils were also increased under the HP-infected surface and therefore these cells were likely involved in the mucosal damage. Enhancement of the expression of HLA-DR antigen was observed in the gastric epithelium with HP infection and it was associated with a significant increase of lymphoid follicles and B cells in the mucosa. In comparison with the HP-infected mucosa of gastritis patients, the number of MNC were increased significantly in the mucosa of both DU and GU patients. The number of HP in the gastric mucosa of DU patients was significantly higher than those of both gastritis and GU patients. In addition, the grade of the infiltration of MNC and IgA positive cells were always greater in DU than in gastritis, regardless of the number of HP. These findings suggested that activation of the local immunity in the gastric mucosa of gastritis and peptic ulcer patients by HP infection may participate in the pathogenesis of gastroduodenal mucosa damage.