Background. Removal and/or 'neutralization' of anti-Galα1-3Gal (αGal) antibodies can prevent or delay the hyperacute rejection of pig organs transplanted into primates. Aim. To determine variations in (1) cytotoxicity to pig kidney (PK15) cells, (2) anti-αGal antibody level, and (3) specificity in adult human (n=46) and baboon (n=38) sera. Methods. Cytotoxicity to PK15 cells was determined by adding rabbit complement to heat-inactivated serum, using a two-color fluorescent dye to distinguish live and dead cells. Anti-αGal antibody level was determined by ELISA using αGal trisaccharide type 2-BSA glycoconjugate as antigen target. Specificity determined by ELISA using four different αGal-BSA glycoconjugates: (disaccharide, trisaccharides type 2 and 6, and pentasaccharide). Results. Cytotoxicity of human AB sera varied from 30-100% PK15 relative cell damage (%RCD), although that of baboon sera of all blood groups varied from 35-100% RCD. In human AB sera, anti-αGal antibody level (at a dilution of 1:80) varied from undetectable to 0.75 (OD at 405 nm), although in baboon sera of all blood groups, anti-αGal antibody level varied from undetectable to >2.0. There was no correlation between anti-αGal antibody level and serum cytotoxicity in either species. Specifically varied among individuals in both human and baboon sera. Conclusions: These studies have demonstrated (1) considerable variation in cytotoxicity and anti-αGal antibody level in human and baboon sera, but a lack of correlation between these two parameters; (2) considerable variation in the specificity of anti-αGal antibodies; (3) blood group B human and baboon sera have lower levels of anti-αGal antibodies; (4) no relation between blood group and specificity of anti-αGal antibodies. Although there are minor differences in the parameters measured, baboons would appear to be suitable surrogates for humans in the pig-to-primate xenograft model.