Background.Natural antibodies (NAbs) against a terminal α1-3 galactosyl(αGal) epitope have been identified as the major human anti-pig NAbs.Methods and Results.We used two synthetic αGal trisaccharides-type 6 (αGal6) and type 2(αGal2)-linked to an inert matrix to remove NAbs from human plasma in vitro. Flow cytometry indicated that an average of 85% of the NAb binding activity was depleted by adsorption with αGal6. By measuring the binding of NAbs to pig peripheral blood mononuclear cells and bone marrow cells, we demonstrated that αGal6 was more effective than αGal2 in removing NAbs, and the combination of αGal6 + αGal2 did not further increase removal of NAbs. The specificity of the removal of NAbs (IgM and IgG) reactive with the αGal epitope by αGal6 matrix was shown by enzyme-linked immunosorbent assay. In vivo studies in nonhuman primates compared plasma perfusion through a αGal6 immunoaffinity column with hemoperfusion through a pig liver for changes in blood pressure, hematocrit, platelets, and NAb adsorption.Conclusions.Both methods reduced the level of anti-pig IgM and IgG xenoreactive antibodies to nearly background, but column perfusion caused less hypotension and reduction in platelets than liver perfusion. Four pig kidneys transplanted into monkeys after column perfusion did not undergo hyperacute rejection, remaining functional for 2-10 days, with a mean functional period of 7 days, demonstrating that a pig kidney can support renal function in a primate.