The role of Campylobacter pylori in weakening the mucus component of the gastric mucosal barrier was investigated. The extracellular material elaborated by C. pylori, cultured from antral mucosal biopsy specimens of patients undergoing gastroscopy, caused a rapid degradation of mucus glycoprotein polymer to glycopeptides comparable in size to those produced by the action of pepsin. Viscosity measurements showed that incubation of C. pylori filtrate with gastric mucus led to a gradual loss in mucus viscosity, which at the end of 48 h decreased by 36%. The C. pylori filtrate also had a detrimental effect on the ability of mucus to retard the diffusion of H+. After 12 h of incubation the permeability of mucus to H+ increased by 10%, whereas a 32% increase in permeability was observed after 48 h. The results suggest that the degenerative changes produced in gastric mucus gel by C. pylori may be a contributing factor in the pathogenesis of gastritis and peptic ulcer.