Blood group ABH antigens are associated with platelets as intrinsic determinants and extrinsically adsorbed antigens, and exist both on glycosphingolipids and on glycoproteins (GPs). We now provide direct evidence that the blood group ABH antigens are prominently associated with platelet GPIb and GPIIb. By immunoprecipitation, a murine monoclonal anti‐A antibody precipitated surface‐biotin‐labelled blood group A1platelet membrane proteins with electrophoretic characteristics identical to those of GPIb/IX and GPIIb/IIIa. By immunoblotting of SDS‐PAGE separated blood group A1platelet proteins the monoclonal anti‐A antibody bound to proteins with electrophoretic characteristics identical to those of GPIb and GPIIb. When immunoaffinity purified GPIb/IX and GPIIb/IIIa, derived from blood group O, A1and A2platelets, were employed for immunoblotting, GPIb and GPIIb only from A1platelets bound the monoclonal anti‐A antibody. By ELISA, wherein monoclonal antibodies specific for GPIb (AP1) and the GPIIb/IIIa complex (AP2) were used to capture and hold antigens from platelet lysate, human anti‐A antibodies reacted with these proteins derived from blood group A1platelets; proteins from blood group A2, O and B platelets showed no reactivity. These results indicate that blood group A antigen is associated with GPIb and GPIIb derived from blood group A1but no