The relationship between Campylobacter pylori infection and gastroduodenal disease was investigated using antral biopsies from 102 patients undergoing endoscopy.C. pylori was isolated from the majority of patients with histologically defined gastritis (89%), gastric ulcer (100%), and duodenal ulcer (94%), but only in 6% of patients with normal gastric mucosa. No obvious epidemiological association between C. pylori infection and recent travel, domestic pets, diet, dental hygiene, drug treatment or symptoms could be established. The susceptibility of 38 of these isolates of C. pylori to 12 antibiotics was determined. The serum anti-C. pylori antibody response was determined by an ELISA technique using an acid extractable antigen on the solid phase, in order to improve the sensitivity and specificity. There was a significant increase in antibody level (p=0.001) of patients with histologically defined gastritis over patients with a normal mucosa. The majority of patients (96%) with a culturable C. pylori infection had raised anti-C.pylori antibodies. However, some patients with neither C.pylori infection nor gastritis had serum antibodies. Possible reasons for this are discussed. The results suggest that a serodiagnostic assay for C. pylori infection may be of value in the assessment of gastroduodenal disease.