Tracheobronchial mucins from healthy individuals and from patients with bronchial asthma or cystic fibrosis (CF) were isolated from lung mucus, purified, and their chemical and physical properties compared. Normal and asthmatic mucins required both a dissociating and a reducing agent for solubilization and exhibited identical chromatographic behavior on Sepharose 4B, Sepharose 2B, and hydroxylapatite and similar amino acid and carbohydrate compositions. In contrast, 1) CF lung mucins were solubilized in the absence of dissociating and/or reducing agents and 2) the majority of the CF mucins analyzed was eluted in the included volume of Sepharose 4B with K d values of 0.3±0.1 rather than in the void volume and thus appeared smaller than normal and asthmatic mucins. The lower molecular weight mucins in CF sputum apparently are produced by bacterial or inflammatory cell proteinases since radiolabelled asthmatic mucin was digested to smaller fragments when incubated with crude CF lung mucosal samples. Furthermore, mucins secreted by tracheal explants from CF and from non-CF individuals eluted in the void volume on Sepharose 4B, suggesting that CF tracheobronchial mucins were not inherently smaller than non-CF mucins.