Antibodies against Campylobacter pylori were determined in 500 blood donors aged 18 to 65 years. Acid extract from a C. pylori strain was used as antigen in enzyme immunoassay. The proportion of donors with high antibody titers increased with age. For IgG antibodies it was 10% in the age group from 18 to 25 years but 60% in the group from 56 to 65 years; the increase for IgA and IgM antibodies was from 5 to 42% and from 7 to 21%, respectively. The geometric mean titers of those with high values showed no clear changes with age, which would imply chronic antigenic stimulus.