Normally the daily volume of lower respiratory tract secretions, in man, is probably less than 100 ml. In hypersecretory disease the volume increases sufficiently to cause cough and expectoration of secretions as sputum. The proportions which are sol or gel vary in disease as does the way in which constituent molecules partition in each phase. The constituent molecules and the cells which produce them (indicated in parentheses) may be classified as follows: 1. Mucus-glycoproteins present as droplets, or sheets (produced by mucous cells), periciliary fluid (serous or ciliated cell or a transudate), surface muco-substance (all epithelial cells) or surfactant hypophase (Clara or type II alveolar cells). 2. Proteins and peptides such as lysozyme (serous cell and macrophage), lactoferrin (serous cell and neutrophil), secretory piece (surface epithelium and submucosal glands), regulatory neuropeptides (dense-core granulated cell and both motor and sensory nerves) and fibronectin (alveolar macrophages). 3. Glycosaminoglycans such as heparan sulphate (epithelial membranes), heparin (mast cell), chondroitin sulphates and hyaluronate (connective tissue constituents). 4. Lipids including triglycerides (stored in cells) glycolipids (cell membrane), phospholipids (type II alveolar cells), sphingolipids (cell membrane), steroids (? Clara cells) and terpenes (cell membrane). 5. Anti-proteases and anti-oxidants such as bronchial protease inhibitors (serous anc Clara cells), alpha-2-macroglobulin (macrophage), alpha-1-antitrypsin (transudate) and anti-oxidants (type II alveolar cell and macrophage). 6. Other 'secretions' including ions and water (surface epithelium and submucosal glands), mediators of inflammation (migratory cell granules and their membranes), and serum proteins (present in transudate/exudate).