The uptake of [14C]-aminopyrine by rabbit gastric epithelial cells was used as an indirect assay for acid secretion from parietal cells. Campylobacter pylori strains, isolated from the stomachs of 3 patients with chronic gastritis, duodenal ulcer, and near-normal mucosa, respectively, inhibited acid secretion as effectively as 10(-4) mol/l cimetidine. The inhibition occurred with whole organisms and with sonicates. Preliminary characterisation suggested that the inhibition was due to a nondialysable protein. The inhibitor was not toxic to gastric epithelial cells. This bacterial product may explain the hypochlorhydria seen in man acutely infected with C pylori.