Background. Mixed lymphohematopoietic chimerism can provide an effective means of inducing long-term immunological tolerance and has been documented in a monkey allograft model. A conditioning regimen including nonmyeloablative or myeloablative irradiation and splenectomy has been used to induce chimerism in a pig-to-primate transplantation model. Since the presence of anti-Galα1-3Gal (αGal) natural antibodies leads to the hyperacute rejection of pig organs transplanted into primates, extracorporeal immunoaffinity adsorption (EIA) of anti-αGal antibodies is also included in the regimen. The effect of the tolerance induction protocol on the anti- αGal antibody response has been assessed. Methods. Anti-αGal antibody was measured after the EIA of plasma through an αGal immunoaffinity column in baseline studies involving two unmodified baboons, one splenectomized baboon, and one baboon that received a challenge with porcine bone marrow (BM), and in three groups of baboons (n=2 in each group) that received different conditioning regimens for tolerance induction. Group 1 received a nonmyeloablative conditioning regimen without porcine BM transplantation. Group 2 received nonmyeloablative conditioning with pig BM transplantation and pig cytokine therapy. Group 3 received myeloablative conditioning, an autologous BM transplant (with BM depleted of CD2+ or CD2+/CD20+ cells), and pig BM transplantation. Results. In the baseline studies, a single EIA of antiαGal antibodies in an unmodified animal initially depleted anti-αGal antibody, followed by a mild rebound. Nonmyeloablative conditioning (group 1) in the absence of pig cell exposure reduced the rate of antiαGal antibody return. Pig BM cells markedly stimulated anti-αGal antibody production in an unmodified baboon (αGal IgM and IgG levels increased 40- and 220-fold, respectively). This response was significantly reduced (to an only 2- to 5.5- fold increase of IgM and IgG) in baboons undergoing nonmyeloablative conditioning (group 2). A myeloablative conditioning regimen (group 3) prevented the antibody response to pig BM, with the reduction in response being greater in the baboon that received autologous BM depleted of both CD2+ and CD20+ cells. No new antibody directed against pig non-αGal antigens was detected in any baboon during the 1 month follow-up period. Conclusions. (i) EIA of anti-αGa1 antibody in unmodified baboons results in a transient depletion followed by a mild rebound of antibody; (ii) exposure to pig BM cells results in a substantial increase in anti-αGal antibody production; (iii) a nonmyeloablative conditioning regimen reduces the rate of antibody return and (iv) markedly reduces the response to pig BM cells; (v) the anti-αGa1 response is completely suppressed by a myeloablative regimen if CD2+ and CD20+ cells are eliminated from the autologous BM inoculum. Furthermore, (vi) challenge with pig BM cells appears to stimulate only an anti-αGa1 antibody response without the development of other (non-αGa1) anti-pig antibodies. We conclude that regimens used for T-cell tolerance induction can be beneficial in reducing the anti-αGa1 antibody response to porcine BM.