Cystic fibrosis (CF) Is characterized by excessive amounts of thick and tenacious mucous secretions that obstruct organ ducts and passages. In the respiratory tract this Is associated with chronic Infection resulting In the hypersecretion of purulent sputum, which the patient finds difficult to clear. We have studied the rheologlc properties of purulent sputum from six patients with CFand five patients with chronic bronchitis to assess whether CF Is associated with Increased sputum viscoelasticity. In addition, we have Isolated the major rheologlc determinants, mucous glycoprotelns, from CF and chronic bronchitis sputa and, using a magnetic micro rheometer, Investigated the possibility that the altered properties of mucus In CF are associated with abnormalities