Mucus and serum are always present in sputum. The concentrations of their markers were measured in mucoid and purulent sputum from patients with chronic bronchitis, asthma, cystic fibrosis, or bronchiectasis. Differences in the concentrations of both mucous and serum glycoproteins were greater among different macroscopic types of sputum than among diseases for the same macroscopic type. Comparison between diseases showed that sputum from patients with asthma had the widest variation in concentrations of markers of mucous glycoprotein and dry macromolecular weight. When the asthma group was further analyzed, it became apparent that patients with extrinsic asthma without chronic bronchitis represented a homogeneous group, whereas those with intrinsic asthma, with or without chronic bronchitis, were responsible for the wide variation. The concentrations of markers of mucous and serum glycoproteins in a sputum sample were correlated with the apparent viscosity of the sputum to establish the relative contribution of the glycoproteins to the viscosity of sputum. In mucoid sputum, the mucous glycoprotein was most important in determining the viscosity, whereas when in infected sputum, both mucous and serum components contributed to the viscosity.