Abstract: An acid inhibitor of bacterial origin which is effective in inhibiting acid secretion from gastric parietal cells; a method of producing this acid inhibitor; a method of administering the acid inhibitor to reduce acid secretion by parietal cells; and a therapeutic composition, which includes the acid inhibitor, for use in the method. In particular, the invention relates to a protein produced by gastric Spirilla, such as Helicobacter pylori (H. pylori), which is effective in inhibiting acid secretion by parietal cells and, thus, can be used in the treatment of peptic ulcer disease and other conditions of the gastroduodenal tract in which there is a tendency for ulceration.