The counts of dendritic cells and cytotoxic T-lymphocytes in the thymus decrease during its aging. The counts of dendritic cells decrease in senile age, while low counts of cytotoxic T-cells are observed only in long-living individuals. Presumably, reduction of the counts of thymic dendritic cells causes disorders in the differentiation of T-cells, particularly of cytotoxic ones, which can represent a mechanism of thymus involution during its aging.