T he Tn-syndrome is an acquired disorder characterized by the polyagglutination of blood cells and the pathological exposure of alpha-N-acetyl-D-galactosamine residues (Tn-antigen) at the cell surface. We now report studies on the platelet of a patient (Ba.) of which 81% reacted positively with a fluorescein conjugate of Helix pomatia agglutinin (HPA). The surface proteins of Ba. platelets were labeled with 125I by the lactoperoxidase-catalyzed procedure; single and two-dimensional electrophoresis on sodium dodecyl sulfate (SDS)-polyacrylamide gels was followed by autoradiography that revealed normal 125I-labeling of the major membrane glycoproteins (GP) but that GP Ib had a faster than normal migration. the abnormal GP Ib of Ba. platelets was strongly labeled when platelet suspensions were treated sequentially with neuraminidase, galactose oxidase, and sodium [3H]borohydride. Unlike the GP Ib of normal human platelets, it was also strongly labeled when Ba. platelets were treated with galactose oxidase and sodium [3H]borohydride alone. Both the alloantigen, PlA1, and quinidine-dependent antibody receptor activity were normally expressed by Ba. platelets, which also bound a monoclonal antibody (AN51) to GP Ib. Analysis of Ba. platelets by crossed immunoelectrophoresis using a rabbit anti-human platelet antibody preparation revealed the presence of an immunoprecipitate in the GP Ib position that had an abnormal appearance and migration in the second dimension. An altered position of the precipitate given by Factor VIIIR:Ag was also noted. Incorporation of HPA into the agarose gel during the first dimension electrophoresis resulted in the specific precipitation of the abnormal GP Ib of Ba. platelets. Our studies show that circulating Tn-platelets contain GP Ib with a modified oligosaccharide chain structure responsible for the platelet expression of Tn-antigen activity.