Most of the mucins isolated from the sputum of patients suffering from cystic fibrosis are acidic. Acidic mucins from a child suffering from cystic fibrosis were degraded by alkali treatment. Analysis of the degradation products demonstrated the wide heterogeneity of the carbohydrate chains linked to the mucin polypeptide moiety. This heterogeneity of carbohydrate chains is probably not restricted to mucins from patients with cystic fibrosis. Bronchial mucins were prepared either from the sputum of different patients belonging to blood group O or B and suffering from cystic fibrosis, chronic bronchitis and other chronic bronchial diseases, or from bronchial washings performed in macroscopically healthy area of the bronchial tree of subjects belonging to blood group O. The chemical composition of each mucin fraction was established and an average carbohydrate chain length was estimated. Acidic mucins isolated from the sputum of 2 children suffering from cystic fibrosis were more sulfated than sialylated and their average carbohydrate chain length was relatively large. These characters were not specific for cystic fibrosis since they were also found in acidic mucins of 2 children suffering from other bronchial diseases. Most of the acidic mucins isolated from the sputum of 3 adults suffering from chronic bronchitis were more sialylated than sulfated and had a relatively short average carbohydrate chain length. The sputum of these patients also contained a variable proportion of neutral or weakly acidic mucins. The mucins isolated from bronchial washings performed in macroscopically healthy areas of the bronchial tree were acidic molecules whose acidic characteristics and average carobhydrate chain length were about the same as for acidic mucins from patients with chronic bronchitis.