Previously we have shown that autoimmune diabetes, induced in rats by a protocol of adult thymectomy and split-dose gamma irradiation, can be prevented by the transfer of a subset of CD4+ T cells with a memory phenotype (CD45RC−), as well as by CD4+ CD8− thymocytes, from syngeneic donors. Further studies now reveal that in the thymus the regulatory cells are observed in the CD25+ subset of CD4+ CD8− cells, whereas transfer of the corresponding CD25− thymocyte subset leads to acceleration of disease onset in prediabetic recipients. However, in the periphery, not all regulatory T cells were found to be CD25+. In thoracic duct lymph, cells that could prevent diabetes were found in both CD25− and CD25+ subsets of CD4+ CD45RC− cells. Further, CD25− regulatory T cells were also present within the CD4+ CD45RC− cell subset from spleen and lymph nodes, but were effective in preventing diabetes only after the removal of CD25− recent thymic emigrants. Phenotypic analysis of human thymocytes showed the presence of CD25+ cells in the same proportions as in rat thymus. The possible developmental relationship between CD25+ and CD25− regulatory T cells is discussed.