Induced expression of the intercellular adhesion molecule 1 (ICAM-1) and of the major histocompatibility complex (MHC) class II antigens has been simultaneously observed on keratinocytes and epithelial cells in the thyroid and kidney, suggesting that ICAM-1 and HLA-DR expression might be under common regulation. We have previously found an association between the presence of Helicobacter pylori and an induced expression of class II antigens on gastric epithelial cells in gastric biopsy specimens from patients with gastritis. In this study we investigated whether ICAM-1 could also be expressed on the gastric epithelium. Thirty-one patients with clinical indications for upper gastrointestinal endoscopy were examined. In 23 patients gastritis was diagnosed endoscopically and histologically and H. pylori was cultured from biopsy specimens. In eight patients neither histological gastritis nor growth of the bacteria was observed. Immunoperoxidase staining demonstrated expression of HLA-DR but not ICAM-1 on the gastric epithelial cells in all patients with H. pylori-associated gastritis, indicating regulatory mechanisms different from those of other epithelial cells.