Bone marrow-derived "passenger" leukocytes that normally reside in the interstitial tissues of whole organs migrate into the recipient after transplantation and establish microchimerism. An equally important phenomenon is the reciprocal migration of circulating recipient leukocytes, which repopulate the interstitium of whole organ allografts. This bidirectional exchange and interaction of bone marrow-derived cells after organ transplantation is considered a seminal event in the acceptance of allografts and in the induction of donor-specific tolerance. The apparent dichotomous role of donor cell chimerism in the processes of organ rejection and acceptance is discussed.