Anti-pig antibodies in human and baboon serum are believed to be directed against α-galactosyl (αGal) epitopes expressed on various pig cells, including vascular endothelia. We have investigated the effect of human sera on the PK15 pig kidney cell line, which abundantly expresses αGal epitopes. To quantitate cell viability, we have used a staining method that differentiates live cells from dead ones. Various carbohydrates (n=28) were added individually to serum at concentrations of 0.125–50 mg/ml. Unmodified serum caused approximate 100% PK15 cell death within 60 min. Carbohydrates that were not αGal based did not significantly protect PK15 cells. Of the αGal-based carbohydrates, only B disaccharide protected PK15 cells from both human and baboon serum (76% and 93% protection, respectively, at 1 mg/ml). Three αGal oligosaccharides provided approximately 80–90% protection against both human and baboon sera at a concentration of 10 mg/ml. Three other closely related structures protected only against baboon serum (> 80%) at high concentration (50 mg/ml), suggesting a difference in anti-pig antibody affinity between baboon and man. Specific anti-αGal antibody-depleted serum caused < 10% pig cell death, whereas the antibodies eluted from the αGal columns caused > 70% pig cell death. In conclusion, this study provides further evidence that (1) αGal structures are the targets for human and baboon anti-pig antibodies, and (2) there may be a therapeutic role for the infusion of specific αGal carbohydrates, or for antibody removal using αGal immunoaffinity columns, in order to prevent hyperacute rejection of pig organs in man. © 1994 by Williams and Wilkins.