The transplantation of donor hematopoietic tissue prior to organ xenografting has the potential to induce lasting T cell tolerance and, possibly, tolerance of natural antibody-producing B cells. However, the development of specific and nontoxic methods of overcoming the immunologic and physiologic barriers to xenogeneic marrow engraftment is a major challenge that must be met before this goal can be achieved. A greater understanding of the species specificity of molecular interactions important for hematopoiesis and cell homing is a first step toward transcending these physiologic barriers. Perhaps most promising is the potential associated with the use of nonprimate xenogeneic donors genetically engineered to make donor tissues more readily capable of surviving and, in the case of hematopoietic cells, competing with host tissues for survival in a human environment.