Recognizes the blood group H type 2 antigens, trisaccharide Fuc伪1-2Gal尾1-4GlcNAc尾1 of human origin. The specificity of this MAb has been identified by haemagglutination tests, immunohistochemistry, binding inhibition studies, and absorption experiments performed with synthetic oligosaccharides. The antibody is virtually nonreactive with H type 1 antigen or with closely related type 2 structures (e.g., Y antigen). A46-B/B10 strongly agglutinates human erythrocytes according to the amount of H substance expressed and can, therefore, easily discriminate between blood groups A1 and A2 as well as A1B and A2B (A1 and A1B are not or only weakly agglutinated). In immunohistochemistry, this antibody seems to provide a highly specific reagent for a restricted number of carcinomas and epithelial lineages in tissue sections and in vitro. This was tested on 753 blood samples. It strongly agglutinates human erythrocytes of blood groups 0, A2 and A2B, but not or only very weakly A1 and A1B. This MAb promises to be useful in the determination of A and AB subgroups in blood group serology. This MAb might also be able to define B subgroups. Among 263 B blood samples, 45 (17.1%) were negative or only weakly positive with A46-B/B10.