The present study was undertaken to understand the mechanism of alteration of the alloreactivity of T cells in aged mice. The experiment utilized bone marrow chimeras revealed that the age-associated changes in alloreactivity are not primarily determined by the bone marrow stem cells but determined by the environment where stem cells differentiate into mature T cells. The limiting dilution analysis and microfluorometry analysis indicated that the changes lie in both frequency of alloreactive T cells and the expression of T cell antigen receptor. Such quantitative and qualitative changes seems to be the causes for declined alloreactivity of T cells in aged mice.