The tracheobronchial secretions from cystic fibrosis patients contained higher levels of protein, DNA and sialic acid than the tracheobronchial secretions from healthy donors. In contrast, the neutral hexose content in CF secretions was strikingly lower than in secretions from normal subjects. The levels of neutral hexose and sialic acid in the CF secretions were found to increase with increasing severity of the disease. The alterations in the levels of these chemical parameters in the secretions of patients with increased disease severity are as a result of increased levels of the mucin content of the secretions, especially of the highly sulfated mucin component. Since mucins are considered, to a large extent, responsible for the viscoelastic properties of the secretions, the enhanced levels of the highly sulfated mucin component in the secretions of the patients with increased disease severity, may contribute to altered rheological properties and hence decreased mucociliary transport of the secretions.