ABO (H) blood group antigens are covalently linked to the oligosaccharide side-chains of von Willebrand factor (VWF). In this study, we investigated the role of the A and B antigens in the expression of VWF adhesive activity. VWF of type A, B or O was purified from fresh frozen plasma. Presence of A or B antigen on the VWF was confirmed by enzyme-linked immunosorbent assay (ELISA) and by immunoblotting with monoclonal anti-A or anti-B. The A or B antigen was also detected in the 48/52-kDa fragment of the respective VWF after trypsin digestion. Removal of A antigen with α-N-acetylgalactosaminidase or B antigen with α-galactosidase did not affect its multimer size or antigenic level, but decreased the ristocetin cofactor (RCoF) activity of the respective VWF by 33–39% (P < 0·01–0·002). Removal of A or B antigen from VWF did not affect the binding of the VWF to immobilized type III collagen. A and B antigens were not detected in platelet VWF. These results indicate that AB structures play a role in platelet aggregating activity of VWF.