['CD4(+)CD25(+) regulatory T (T(reg)) cells develop in the thymus and can suppress T cell proliferation, modulated by Foxp3 and cytokines; however, the relevance of CD44 in T(reg) cell development is less clear. To address this issue, we analyzed Foxp3 expression in CD44(+) T(reg) cells by using multiple parameters, measured the levels of the immunoregulatory cytokine interleukin (IL)-10 in various thymocyte subsets, and determined the suppressor activity in different splenic T(reg) cell populations.', 'Within mouse thymocytes, we detected T(reg) cells with two novel phenotypes, namely the CD4(+)CD8(-)CD25(+)CD44(+) and CD4(+)CD8(-)CD25(+)CD44(-)staining features. Additional multi-parameter analyses at the single-cell and molecular levels suggested to us that CD44 expression positively correlated with Foxp3 expression in thymocytes, the production of IL-10, and T(reg) activity in splenic CD4(+)CD25(+) T cells. This suppressive effect of T(reg) cells on T cell proliferation could be blocked by using anti-IL-10 neutralizing antibodies. In addition, CD4(+)CD25(+)CD44(+) T(reg) cells expressed higher levels of IL-10 and were more potent in suppressing effector T cell proliferation than were CD4(+)CD25(+)CD44(-) cells.'] Does cD44 expression positively correlate with Foxp3 expression and suppressive function of CD4+ Treg cells?