The aggregation of lymphoid cells in the bronchial mucosa has been named the bronchus-associated lymphoid tissue (BALT) and investigated in comparison with the gut-associated lymphoid tissue (GALT). To elucidate precisely the structure and function of the BALT, the present study examined the age-related change in the mouse BALT by light microscopy. We also observed the characteristics of the overlying epithelium, especially the lectin-binding properties of the epithelial cells, by the combined use of light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).