We studied a 5-year-old girl presenting with a markedly prolonged bleeding time. Her platelets were refractory to collagen stimulation, but the response to other agonists was normal. There were no coagulation abnormalities as measured by standard tests. Two-dimensional electrophoresis showed no abnormalities of the patient's platelet membrane glycoproteins. When the patient's plasma or purified plasma IgG was mixed with normal platelets, collagen-induced platelet aggregation was blocked. Western blotting showed the presence of an antibody in the patient's plasma directed against a protein of molecular weight 85 to 90 Kd under both reducing and nonreducing conditions. This protein comigrated with glycoprotein (GP) IV immunoprecipitated by OKM5 from 125I-labeled platelets. Immunoprecipitation of 125I-labeled normal platelet glycoproteins with the patient's IgGs also yielded an 85- to 90-Kd protein that migrated on the diagonal following nonreduced/reduced two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Despite similarities in electrophoretic behavior, the antigen was not demonstrated to be GPIV, since purified GPIV was not recognized by the antibody.