The tonsil has its maximum size in early childhood and shows a marked shrinkage in adulthood. This study examined human tonsils by histological and immunohistochemical methods to compare their fundamental structures and the distributions of a few subsets of the lymphocytes between both age groups. Avidin-biotin-peroxidase complex technique was employed as immunohistochemical method, and anti-IgM antibody, anti-IgA antibody, CD4 and CD8 monoclonal antibodies were used to see the distribution of B lymphocytes, IgA-containing cells helper and suppressor T lymphocytes in the tonsillar tissue, respectively. Adult tonsils were revealed to have much smaller areas of the lymphoid follicles and germinal centers than those of children(p<0.01). Immunohistochemical examination showed that the number of IgM-positive B lymphocytes was much smaller in the interfollicular area of the tonsils from adults than those from children(p<0.01). IgA-positive cells were mostly found to be plasma cells. The numbers of these cells were not so different in both groups, suggesting the IgA-producing activity is still present in adulthood. The mean numbers of helper and suppressor T lymphocytes were greater in adult tonsils, but this difference was not statistically significant. The ratio of these two lymphocyte subsets was identical (2 : 1) in both age groups.