To define the immunocytochemical localization of secretory component (SC), IgA and J chain in human bronchioles and alveoli, a direct peroxidase-labeled antibody method was used. SC was found in non-ciliated cells of the bronchioles including respiratory bronchioles and type II alveolar epithelial cells, whereas SC was rarely present in ciliated cells and type I alveolar epithelial cells and was absent from goblet cells. In the positively reacting cells, SC was found in secretory protein synthetic organelles such as perinuclear spaces and endoplasmic reticulum, Golgi complexes, and on the external surfaces of the apical and basolateral plasma membranes. IgA and J chain were localized in the epithelial cells where SC was found. Ultrastructually IgA was present on the apical and basolateral plasma membranes, in pinocytic invaginations of the membranes, and in vesicles distributed through the cytoplasm, especially in the apical cytoplasm of the epithelial cells where SC was found. In addition, IgA and J chain were found to be associated with the endothelial cells of the capillaries, plasma cells and the surrounding interstitium. These observations suggest that SC is synthesized and secreted by epithelial cells, especially non-ciliated cells of the bronchioles including respiratory bronchioles and type II alveolar epithelial cells. They also suggest that secretory IgA (sIgA) is transported into alveolar spaces and the bronchiolar lumen through these cells by SC-mediated transport mechanism. This sIgA may play an important role in defense mechanisms of the lower respiratory tract and alveoli.