The mode of action of colloidal bismuth subcitrate (CBS), a colloidal bismuth preparation often used in the treatment of gastroduodenal ulcers, is unknown. The precipitation characteristics of CBS were determined by titration with hydrochloric acid and gastric juice. The precipitation pH were 4.0 and 3.5 respectively. Preoperative administration of CBS resulted in a preferential accumulation of bismuth in chronic gastric ulcers as determined by tissue bismuth assays. CBS and gastric glycoprotein formed an in vitro complex which markedly retarded the migration of H+. One of the mechanisms by which CBS promotes ulcer healing is through its behaviour as a diffusion barrier to hydrochloric acid.