Light scattering has been used to investigate the structure of human tracheobronchial mucin glycoproteins (HTBM) from the sputum of cystic fibrosis patients. The specimen was extracted using 6M guanidinium hydrochloride solution and fractionated by gel exclusion chromatography on Sephacryl S-1OOO. The fractionated HTBM was purified by density gradient ultracentrifugation. Purity of the resulting material was confirmed by SDS polyacrylamide gel electrophoresis and uv spectroscopy. Light scattering measurements on the fractionated mucins yield weight-average molecular weights M , , and z-average radii of gyration Rg, *. The native cystic fibrosis HTBM consisted of a high molecular weight fraction with M , = 9.3 x lo6 daltons and a lower molecular weight fraction containing partly degraded mucins. After reduction and carboxymethylation of the high molecular weight native fraction, the resulting material was separated into three pools with M , values of 5.1 X lofi, 1.6 X lo6, and 400,000. The derived molecular weights for the protein cores M,,,, and the experimental radii of gyration are found to be consistent with the M,, , -R , relation established previously for submaxillary, cervical, and gastric mucins. These results imply that HTBM has the same extended-coil conformation reported for other mucins and has a molecular structure consisting of subunits, linked into linear chains via covalent (disulfide) bonds.