Mucous secretions were collected from tracheas of patients undergoing minor surgery under general anaesthesia with tracheal intubation, and mucus glycoproteins were isolated by using isopycnic density-gradient centrifugation in CsCl/guanidinium chloride. 'Whole' mucins were excluded from a Sepharose CL-2B gel, whereas subunits obtained after reduction were included. Trypsin digestion of subunits afforded high-Mr glycopeptides (T-domains), which were further included in the gel. The latter fragments are heterogeneous and comprise two or three populations, as indicated by gel chromatography and ion-exchange h.p.l.c. Rate-zonal centrifugation showed that the 'whole' mucins are polydisperse in size, with a weight-average Mr of (14-16) x 10(6). The macromolecules were observed by electron microscopy, as linear and apparently flexible thread-like structures. Subunits and T-domains had weight-average contour lengths of 490 nm and 160 nm respectively. It is concluded that mucus glycoproteins are present in secretions from the healthy lower respiratory tract. The 'whole' tracheal mucins are assembled from subunits, which in turn can be fragmented into high-Mr glycopeptides corresponding to the oligosaccharide domains typically found in mucus glycoproteins. The size and macromolecular architecture of the tracheal mucins is thus similar to that observed for mucins from human cervical mucus, chronic bronchitic sputum and pig stomach, providing yet another example of this general design of these macromolecules, i.e. subunits assembled end-to-end into very large linear and flexible macromolecules.