We investigated the abnormal small-particle spike discerned in the platelet-rich plasma of patients with severe idiopathic autoimmune thrombocytopenic purpura. By electron microscopy, erythrocyte as well as platelet fragments were found in the 27,000 X g plasma sediment of 15 patients with severe disease. These fragments were not observed in the plasma sediment of 12 normal subjects, two healthy asplenic subjects, three patients with thrombocytopenia of nonimmunologic origin, and two with autoimmune thrombocytopenic purpura in remission. Weak complement sensitization of red blood cells was noted in seven out of 12 patients with the disease. Coating of red blood cells with IgG or IgM was not detected in these patients. Whereas erythrophagocytosis was conspicuously absent, phagocytosis of intact platelets as well as platelet fragments and other cellular debris was frequently observed. Autoimmune mechanisms may be directed against erythrocytes as well as platelets in most cases of severe idiopathic autoimmune thrombocytopenia.