The role of Campylobacter pyloridis, a spiral bacteria associated with gastritis and peptic ulcers in weakening the mucus component of gastric mucosal barrier was investigated. The colonies of bacteria, cultured from antral mucosal biopsies of patients undergoing gastroscopy, were washed with saline, passed through sterilization filter and the filtrate was examined for protease and glycosylhydrolase activities. The obtained results revealed that the filtrate exhibited a strong proteolytic activity not only towards the typical protein substrates such as albumin but also towards gastric mucin. Optimum enzymatic activity for degradation of mucin was attained at pH 7.0 and the protease activity was found in a low m.w. (less than 50K) protein fraction. The filtrate showed little glycosylhydrolase activity and did not cause the hydrolysis of mucin carbohydrates. The data suggest that C pyloridis infection weakens the gastric mucosal defense by causing proteolytic degradation of mucin component of the protective mucus layer.