Antibody activities to campylobacter pylori in serum were estimated by an enzyme-linked immunosorbent assay (ELISA) to crude antigens, prepared by sonication of whole organisms obtained from bacterial culture in 100 patients with chronic gastritis. Significantly raised serum IgG antibody activities to C. pylori was found in colonised patients with gastritis, especially in patients with active gastritis. High activities were also found beyond the age of 30. In 6 patients cleared of C. pylori with furazolidone and/or colloidal bismuth subcitrate (De-NoL), serial testing has shown a fall in activity to normal level by 1/2 year in 4 patients. The specificity and sensitivity of the sero-diagnostic assay was 85.3% and 97% respectively. The positive and negative predictive values were 92.8% and 93.5% respectively. The results indicate that such a serodiagnostic assay could be used to screen patients with C. pylori colonisation in epidemiological surveys.