The histological features of non-autoimmune chronic gastritis are consistent with those of a local immune response by the gastric mucosa. If Campylobacter pylori is causative in this condition, then the immune response will be in part specific for this organism. Antibodies to C. pylori are present in the gastric juice of infected patients. Coating of mucosal C. pylori by antibody in vivo can be demonstrated by immunohistochemistry, and a neutrophil infiltrate (active gastritis) is associated with coating by opsonising antibodies. Gastric mucosa from patients with C. pylori produces specific immunoglobulin in vitro, and this antibody production correlates with the plasma cell infiltrate seen in these patients. Lymphoid follicles in gastric mucosa, implying local antigenic stimulation, are a specific feature of campylobacter-associated gastritis. The inflammation gradually abates following eradication of the bacteria. We conclude that the morphological features of campylobacter-associated gastritis represent the immune response of the mucosa to C. pylori.