The role of mucous glycoproteins in the rheologic properties of cystic fibrosis sputum. 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 rheologic properties of purulent sputum from six patients with CF and five patients with chronic bronchitis to assess whether CF is associated with increased sputum viscoelasticity. In addition, we have isolated the major rheologic determinants, mucous glycoproteins, from CF and chronic bronchitis sputa and, using a magnetic microrheometer, investigated the possibility that the altered properties of mucus in CF are associated with abnormalities in these glycoproteins. Creep compliance analysis indicated that the CF sputa possessed raised levels of both elasticity (p less than 0.01) and viscosity (p less than 0.01). These increases in both rheologic parameters were found to be associated with increases in the DNA content (p less than 0.01) and dry weight (p less than 0.05). Mucous glycoproteins were isolated from CF and chronic bronchitis sputum samples by gel filtration on Sepharose CL4B, followed by concentration to form 8% wt/wt gels. In the absence of other sputum components, no abnormality in the rheologic properties of CF mucin gels could be detected. However, when DNA was added, the CF gels responded with increases in both elasticity and viscosity of as much as 30% (p less than 0.05), an effect not observed in the chronic bronchitis gels. These results suggest that a subtle abnormality may exist in CF mucous glycoproteins and that this could have a role in the altered physical properties of mucous secretions in CF. 