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It is well known that leukocyte composition, T cell phenotypes and immune function change in aged mice and humans. However, limited and conflicting results on the age-related immune changes in middle-aged mice were reported. Identification of the characteristics of allogeneic immune responses in aging mice may offer important information for transplantation immunology. The major age-related changes in the immune cell phenotypes and function of 12 months old mice include: 1) the significantly decreased CD4+ cell population in the peripheral blood, the major peripheral CD4+ cells is CD45RBlowCD62Llow memory phenotype; 2) the in vitro responses to alloantigens and Con A of splenocytes markedly reduced; 3) the in vivo secondary humoral immune responses to alloantigens significantly declined; 4) the age-related alteration in the thymus mainly occurred in CD4/CD8 double positive (DP) stage; and 5) increased CD80+ and MHC class Ⅱ+ cell population in spleens. Thus, the major age-related immune changes in 12 months old mice occurred in CD4+ T cells in the periphery and DP stage in the thymus, which may subsequently lead to the decreased allogeneic immune responses and the different sensitivity to immunosuppressive drugs and treatments. Further studies on the characteristics of allogeneic immunity in aging individuals may help to determine the appropriated treatment for transplant aging individuals. Cellular & Molecular Immunology. 2004; 1(6) :440-446.