A critical revision of the data in the literature regarding mixed allogeneic systemic chimerism by transplantation permits to perceive the aspects of undoubted interest in the immunological and clinical field. The usefulness of the cellular exchange, with the aim of the acceptance of the transplantation , that is carried out between recipient and the transplanted organ and which constitutes a completely peculiar immunological situation, comparable to the immunotolerance of the fetal period even though with same not totally identificable is pointed out.