Gastric/duodenal biopsy material from 52 patients was examined immunohistochemically for Helicobacter (Campylobacter) pylori. Specimens from 34 of the patients harboured Helicobacter pylori along the mucosal surface and 13 of these featured, in addition, immunopositive material within the lamina propria. The remaining 18 biopsies were non-reactive. This observation suggests that Helicobacter pylori can penetrate the epithelium and its basement membrane, resulting in the production of specific systemic antibodies.