P reformed and elicited Ab’s against the Galα1,3Gal terminating carbohydrate chains (αGal Ab’s) are the primary cause of hyperacute and acute vascular xenograft rejection in pig-to-primate transplantation. αGal Ab’s are produced by long-lived Ab-producing cells that are not susceptible to pharmacological immunosuppression. We reasoned that antigen-specific elimination of αGal Ab’s might be achieved in vivo by systemic administration of nonimmunogenic polyvalent αGal structures with high avidity for αGal Ab’s. We devised GAS914, a soluble trisaccharide-polylysine conjugate of approximately 500 kDa that effectively competes for αGal binding by αGal IgM (IC₅₀, 43 nM) and IgG (IC₅₀, 28 nM) in vitro. Injections of GAS914 in cynomolgus monkeys, at the dose of 1 mg/kg, resulted in the immediate decrease of more than 90% of circulating αGal Ab’s and serum anti-pig cytotoxicity. In baboons, repeated injections of GAS914 effectively reduced both circulating αGal Ab’s and cytotoxicity over several months. Studies with [¹⁴C]GAS914 in rhesus monkeys and Gal^–/– mice indicate that GAS914 binds to circulating αGal Ab’s and that the complex is quickly metabolized by the liver and excreted by the kidney. Remarkably, posttreatment αGal Ab titers never exceeded pretreatment levels and no sensitization to either αGal or the polylysine backbone has been observed. Furthermore there was no apparent acute or chronic toxicity associated with GAS914 treatment in primates. We conclude that GAS914 may be used therapeutically for the specific removal of αGal Ab’s.