A young women affected by Hodgkin's disease developed chronic autoimmune thrombocytopenic purpura. Splenectomy induced normalization of her platelet count, but hemorrhagic symptoms did not disappear. The patient's platelets did not aggregate in response to collagen and ADP and the IgG fraction of the patient's plasma induced the same defect in normal platelets. The women's IgG recognized glycoproteins IIb and IIIa of normal platelet membranes. Prednisone therapy induced the disappearance of bleeding symptoms and the normalization of platelet aggregation.