BACKGROUND: Pigs are a potential source of red blood cells (RBCs) for transfusion into humans, but the presence of galactose-α1,3-galactose (Gal) epitopes on their surface, against which humans have anti-Gal, has been perceived as a major barrier, α1,3-Galactosyltransferase gene-knockout pigs, which do not express Gal epitopes on RBCs (Gal-/-), have recently become available. STUDY DESIGN AND METHODS: In vitro, RBCs from Gal-/- pigs were exposed to sera from naïve humans or baboons or from baboons previously sensitized to pig antigens; immunoglobulin binding was measured by flow cytometry, and cytotoxicity, by a hemolytic assay. In vivo, relatively small numbers of Gal-/- RBCs were transfused into two nonsensitized untreated baboons. The survival of pig RBCs was detected by flow cytometry. RESULTS: In vitro, binding of immunoglobulin (Ig) M from naïve human or baboon sera was detected to Gal-/-RBCs but was significantly less than to Gal+/+ RBCs; IgG binding to Gal-/- RBCs was absent or minimal. Sera had minimal cytotoxicity to Gal-/- RBCs compared to Gal+/+ RBCs. Sensitized baboon sera demonstrated much higher IgG binding to Gal-/- RBCs and increased cytotoxicity, but again these were less than to Gal+/+ RBCs. In vivo, the transfusion of relatively small volumes of Gal-/- RBCs was followed by detection of the cells in the baboon's blood for only 5 minutes. CONCLUSION: Pig RBCs are rapidly phagocytosed from the primate circulation by a mechanism not involving anti-Gal.