UNLABELLED: It has been clearly established that Helicobacter pyloni (H. pylori) play an important role in the pathogenesis of some chronic diseases of upper gastrointestinal tract. A lot of attention to the complicated immunological processes induced by the infection was paid. The clinical outcome of the damage of gastric mucosa by H. pylori depends on the type and the intensification of these processes. During many years the acquired (specific) immunological response on the infection was analyzed by scientists, but much more researches on innate defense was done lately The mast cells constitute some important parts of the immunological innate defense. H. pylori colonization of gastric mucosal surface elicits a conspicuous infiltration comprising of lymphocytes, and plasma cells as well as neutrophils, eosinophils, macrophages and the granule cells of connective fissue. Some difficulties with the mastocytes visualization in conventional histological slides in light microscope caused so far the little attention of their participation in chronic gastritis. The aim of the study was the assessment of the mastocytes participation in the infiltration of immunological cells induced by H. pylonri in chronic gastritis. MATERIAL AND METHODS: The subjects were twenty dyspeptic children aged 9-17 years underwent upper GI endoscopy procedure. Twelve children were H. pylori positive. Remaining eight children H. pylori negative composed a control group. Gastric antrum and corpus tissue specimens in the conventional (light) and electron microscopy were examined. The specimens intended for electron microscope assessment were fixed in the solution of 1% glutaraldehyde and 2.5% paraformaldehyde at 40 C (pH 7.4) for 24 hours, and postfixed in 2% osmium tetroxide at the same conditions. Ultrathin sections were contrasted with uranyl acetate and lead citrate. Results. Ultrastructural analysis revealed two distinct, morphological forms of mast cells in gastric mucosa. All mast cells contained multiple granules with fine-grained material but their appearance was distinct in both types of these cells. Statistical analysis revealed that the count of mast cells in gastric mucosa was increased in H. pylori positive when compared with H. pylori negative children. It was also affirmed that in specimens from H. pylori infected children mast cells more frequently were seen in the gastric epithelium.