Xenotransplantation could potentially overcome the organ shortage that currently limits the field of transplantation. Because of their breeding characteristics as well as their size and physiologic similarities to humans, we have chosen miniature swine as possible xenograft donors, and are currently attempting to develop a means of using mixed xenogeneic chimerism as an approach to tolerance induction in swine-to-primate, species combinations. One major barrier to organ grafting from pig to man is the presence in human serum of preformed natural antibodies (NAb) reacting with antigens expressed on porcine endothelial cells and causing hyperacute rejection. Previous experiments performed in our laboratory have shown that both humoral and cellular tolerance can be induced in a concordant xenogeneic species combination (rat → mouse) using donor bone marrow infusion following conditioning with a nonmyeloablative regimen. Induction of chimerism in these animals was associated with a marked reduction in the level of IgM natural antibodies that recognize rat bone marrow cells. A similar approach could also lead to humoral and cellular tolerance induction in the swine → human species combination, permitting transplantation of vascularized organs from the swine donor. To determine the potential of bone marrow transplantation to induce a state of “natural antibody tolerance,” it was essential to determine whether or not all human NAb target antigens expressed on swine EC are also expressed on cells derived from swine bone marrow. We have addressed this question by evaluating the ability of various swine bone marrow-derived cells to absorb human IgM and IgG NAb that bind to swine EC. Our results demonstrate that swine bone marrow cells and their progeny can absorb almost all IgM NAb that bind to swine EC, as detected by flow cytometric and ELISA assays. Specificity of absorption was demonstrated, as total serum IgM levels declined only minimally after absorption on swine BMC and to an extent comparable to that observed following absorption with human cells, which did not deplete swine EC-binding NAb. Human IgG binding to swine EC was also completely absorbed by swine BMC. These results suggest that a state of “NAb tolerance” could be induced by successful swine marrow engraftment in man. Furthermore, swine PBL, platelets, and EC were able to absorb most IgM NAb that bound to swine BMC, suggesting that absorption using antigen from any of these tissues might facilitate marrow engraftment, and hence tolerance induction, in this species combination.