The Clara cells are a group of cells, sometimes called "nonciliated bronchiolar secretory cells", found in the bronchiolar epithelium of mammals including man, and in the upper airways of some species such as mice. Their secretory function is assumed from their ultrastructural appearance, that usually includes copious smooth endoplasmic reticulum, many apical mitochondria and scanty secretory-like dense vesicles near the luminal membrane. An apical cap of the cell usually bulges into the airway lumen, and secretion may be by shedding this cap, or by diffusion secretion or by merocrine secretion in individual granules. The chemical nature of the secretion probably includes protein, glycoprotein and lipids. The secretion may contain enzymes. Its function is presumably to determine the chemical and physical properties of the lining of small airways, and it could behave as a kind of bronchiolar surfactant, limiting lung collapse. The Clara cells also contain much cytochrome P450 dependent mixed-function oxidases, which presumably play a detoxifying role. It is not known whether these oxidases can be secreted or whether they have a lipid-synthesizing function. Clara cells may be important in human disease, both by giving rise to tumours and by taking part in metaplastic changes in bronchiolar disease.