Changes in the glycosylation in cancer may lead to an aberrant expression of A, B incompatible or xenogeneic blood group related antigens. To characterize the specificity of IgG antibodies to A, B, and related glycans in sera of gastrointestinal cancer patients, serum probes and affinity-isolated antibodies were analyzed in the indirect and competitive ELISA using a set of homogenous polyacrylamide (PAA) glycoconjugates. Monoreactive antibodies recognizing A di (I) and cross-reactive antibodies to A di /B di /B tri (II) or A di /A tri /Fs di /Core5 (III) were affinity-isolated on A di -PAA-Sepharose. The population I showed a higher affinity to A di -PAA than cross-reactive antibodies. The antibodies II were more specific to B di and may belong to the core alpha-Gal reactive antibodies but are also capable of recognizing A di . The antibodies III were more specific to A tri ; they agglutinated A-erythrocytes and belong to anti-A isoantibodies reactive to xenogeneic oligosaccharides. The purified antibody samples were non- or faintly reactive to Tn. The IC 50 values of PAA glycoconjugates ranged from 6 × 10 −8 to 7 × 10 −6  M. No or weak binding of antibodies to the unrelated antigens used in the detection of polyreactivity (ferritin, casein, and DNA) was observed.