The biochemical and rheological analysis of sputum allows one to detect abnormalities of the bronchial secretion and to describe the development of the bronchial disease. The main biochemical components of sputum and their biological activities are first described. The fibrillar structure of the mucus is related to heavy weight glycoproteins and these macromolecules contribute to the viscoelastic properties of the bronchial secretion, by their capacity to combine with other proteins such as secretory immunoglobulin A (S-IgA). As the chronic bronchial hypersecretory state progresses, the glandular synthesis of mucins and/or secretory proteins decreases while the passive filtration of blood-derived proteins, such as serumalbumin, increases. Such biochemical changes are frequently associated with a rise in viscosity and a loss in elasticity of sputum. These rheological modifications result in an impairment of the mucociliary clearance and therefore contribute to airway obstruction. The assessment of different indices such as the equilibrium in mucin secretion, S-IgA/serum-albumin ratio, viscosity and elasticity of sputum and its mucociliary transport rate, should lead to a better knowledge of the defense properties of the bronchial mucosa.