CD45 is known to have tyrosine phosphatase activity for signal transduction of T cells. Immunomodulation of CD45 has been tried to prevent T cell-mediated graft rejection in organ transplantation. In vitro study showed that blockade of CD45RB, an alternative splicing isoform of CD45, inhibited proliferative response of T cells after allogeneic stimulation. Treatment with a monoclonal antibody (mAb) against CD45RB induced long-term allograft acceptance in some mouse organ transplantation models. In a rat heart allograft model, a single injection of anti-rat CD45 (RT7) mAb which bound to allomorphic region of RT7 also induced allograft acceptance. CD45/RT7 is also a useful tool of targeting hematopoietic cells, because of the selective expression on all hematopoietic cells. There are two allomorphic forms of CD45 (RT7 a and RT7 b ) in the rat. Using RT7 system, a rat heart allograft model from RT7 a donors to RT7 b recipients was designed to test functional relevance of graft-associated hematopoietic cells (microchimerism) to allograft acceptance. Then donor-derived hematopoietic cells were selectively depleted using anti-RT7 a mAb in vivo. Depletion on day 0 prevented allograft acceptance and was associated with severe acute or chronic graft rejection, while depletion on day 18 after transplantation showed no effect. This experimental study showed a crucial role of microchimerism in induction phase of allograft acceptance. In conclusion, the CD45/RT7 system is not only a target molecule for tolerance induction, but also an useful tool for experimental models in transplantation immunology. In this review, we introduce basic properties of CD45 and recent results with manipulation of CD45.