The changes of two functions of regulatory T cells in mice of different ages were determined. Mice were immunized with SRBC. DTH responsiveness of spleen cells and the production of IL-2 after Concanavalin A stimulation of the same cell suspension was measured. Cell populations were analysed for expression of Lyt-2 and Thy-1 surface markers. The relationship between DTH responsiveness and T cell subset distribution in peripheral blood was analysed. A direct relationship between age related changes in DTH responsiveness and T cell subset distributions in peripheral blood was observed in most individual animals.