Necropsy gastric mucus infected with Helicobacter pylori has a reduced capacity to concentrate colloidal bismuth subcitrate when compared with non-infected mucus. Mucus mounted in a modified in vitro diffusion chamber was bathed with colloidal bismuth subcitrate solutions at different concentrations and pH levels. Bismuth was measured by atomic absorption spectrophotometry to assess intramucus colloidal bismuth subcitrate concentrations. Bismuth concentrations in non-infected mucus were higher than in Helicobacter pylori infected mucus at all experimental colloidal bismuth subcitrate concentrations and pH levels. Regardless of the infection status, the intramucus concentration of colloidal bismuth subcitrate was dependent upon the concentration of the bathing solution and independent of the pH and the mucus thickness. Colloidal bismuth subcitrate solubility in saline solution varied with pH, and was least soluble in the pH range 1.1 to 3.25 and more soluble above and below this pH range. This study suggests that Helicobacter pylori infection is associated with physicochemical changes in the gastric mucus with a reduction in its capacity to concentrate colloidal bismuth subcitrate. Such a reduction may compromise the attainment of optimum colloidal bismuth subcitrate concentrations necessary for its bactericidal activity.