The lower respiratory tract of the hamster was examined for evidence of endogenous peroxidase activity. Using the standard diaminobenzidine cytochemical technique with controls to distinguish peroxidase from other hemoproteins, brown peroxidase reaction product was observed in the tracheal lumen and within epithelial secretory cells. The lumen and secretory cells of submucosal glands also contained peroxidase activity. Peroxidase-positive cells were most numerous in the upper trachea. Activity gradually decreased distally so that the least number of positive cells occurred in the extrapulmonary bronchus. Older animals contained many more positive cells than did younger animals. Within the lung, all epithelial cell types in both conducting and respiratory zones lacked activity. Peroxidase-positive cells in the tracheo-bronchial epithelium were identified as mucous cells by electron microscopy. Within these cells, peroxidase activity was found in the nuclear envelope, cisternae of rough endoplasmic reticulum, Golgi saccules, condensing vacuoles, and secretory granules. Discharge of the granules into the lumen appeared to result from a merocrine type of secretion. These ultrastructural findings are similar to those described for the secretory peroxidase in mammary and salivary glands. The peroxidase in these glands plays a key role in a nonspecific antibacterial system. Although the function of airway peroxidase is presently unknown, it is quite possible that it too possesses anti-infectious properties, thus forming an important adjunct to the well-known physical, cellular, and immunologic processes that protect the respiratory tract from microbial and toxic injury.