Abstract Mucins of bronchial fibrillar mucus have been isolated from sputum of a child with cystic fibrosis. Two fractionation procedures have been studied comparatively: —Gel chromatography on Sepharose 4B followed by preparative liquid film high-voltage electrophoresis does not allow to prepare mucins cleared of nucleic acids, immunoglobulins A and bronchial transferrin. —Ecteola cellulose chromatography and further Sepharose 4B gel chromatography result in the isolation of bronchial mucins with acidic character. Immunological and chemical studies demonstrate the purity of the mucins thus obtained. Serine + threonine residues represent more than 40% of the polypeptide moiety. These mucins contain more than 70% carbohydrate consisting of hexosamines, galactose and fucose. Acidic character is due to the presence of both sialic acid residues and sulfate groups.