Ultrastructural studies of the rabbit bronchial mucosa were performed with scanning and transmission electron microscopy. The former revealed that the bronchus-associated lymphoid aggregates could be distinguished from the surrounding tissue, since they were covered with epithelial cells possessing microvilli but not cilia. These lymphoepithelial cells were irregularly placed and provided crevices of varying dimensions at their junctions with other cells. On transmission electron microscopy, these cells showed scanty cytoplasm, several mitochrondria, few other organelles and only rare phagosomes. Just below the epithelium in the lymphoid aggregates, basophil-like cells were seen regularly and were also found in the lymphoepithelium and adjacent normal bronchila epithelium, here lying between cells above the basement membrane. The possible origin of these cells and their potential relationship to lung defense and hypersensitivity disease are discussed.