With a clear and straightforward experimental design of donor chimerism from skin grafts in mice, the authors show the two opposing roles of microchimeric cells, which were mainly donor T cells. In summary: donor microchimeric T cells induce tolerance only in immunologically immature hosts, whereas in others microchimerism results in immunity. However, if the graft expresses antigens that are not expressed by chimeric cells, rejection results. Donor T cells can migrate to the host thymus and induce tolerance. Chimeric donor T cells are unable to tolerize mature naive T cells, and in these conditions donor T cells are immunogenic. In conclusion, the presence of chimeric cells, mainly donor T cells, can lead to acceptance or rejection of the graft.