Three cell types contribute to the bronchial mucus srcritions - the goblet cells of the surface epithelium and the mucous and serous cells of the submucosal glands. while chemical techniques are at present unable to separate the secretions of the three cell types, histochemically it is possible to study individual cells and identify sites of sialic acid and sulphate, the only two acidic groups known to occur in mammalian mucus. The aim of the present study has been to identify, using histochemical and autoradiographic techniques, the varieties of acidic mucins produced by the three cell types and to investigate their development in the foetus and child} the possible variation in disease} the control of cellular secretion,and to reproduce in experimental animals the alterations seen in disease. A quantitative approach, using a point counting technique, has been applied to histochemical results for the first time and has enabled reliable comparisons of different histochemical techniques, and comparison between normal bronchi and those affected by disease. The results show that a wider range of acidic mucins are produced by goblet and mucous cells for the bronchus than have previously been reported for this or for any other site. The serous cell secretion is apparently a complex carbohydrate with both sulphate and sialic acid, though the histochemical behaviour of these acidic groups differs from that in mucous cell secretion. Supporting evidence for the hi3tochemical results has been obtained by tracing the uptake and intracellular behaviour of radioactive metabolites in human bronchial glands using an in vitro organ culture technique, and from biochemical estimation of sialic acid and sulphate in whole sputum. In cystic fibrosis and in hypersecretory states the alterations in acidic mucins appear to be quantitative rather than qualitatively different in nature. Histochemical changes similar to those seen in human hypersecretory states can be produced in the rat tracheo-bronchial tree by exposure to irritants. Adenocarcinomas and 'alveolar cell carcinomas of the lung produce a range of acidic mucins which vary with the histological pattern and the degree of differentiation of the tumour.