The two basic proteins lysozyme and lactoferrin have been isolated from solubilized mucoid sputum from patients with chronic bronchitis in one step by cation exchange chromatography. In sputa from 13 patients with chronic bronchitis their mean concentrations were 0.4 g/l and 0.7 g/l, respectively, representing 6.6% and 11.5% of the total amount of solubilized protein. Lysozyme and the acid mucin glycoproteins of sputum formed aggregates at low ionic strength, probably as a result of electrostatic interactions between the two. Although only aggregates were formed and not a viscoelastic fluid or a gel, these interactions may contribute to the viscoelastic properties of native sputum.