The neuraminic acid content of sputum from 48 men with early chronic bronchitis has been estimated in samples collected over a period of three years. The results are compared with those from 29 advanced bronchitic patients and are related to the clinical features of both groups and to the physical and biological properties of the sputum. A seasonal variation in neuraminic acid content has been noted for the first time with higher levels during the winter months. Clinical assessment of sputum pourability correlated well with measured viscosity. The viscosity of mucoid sputum was related to its neuraminic acid content but also to the yield of dry macromolecular material. In the early bronchitic group whose sputum was assessed for purulence at monthly intervals pus was more often present in those men whose mucoid sputum contained higher levels of neuraminic acid. These findings are discussed in relation to the cause of exacerbations of chronic bronchitis.