Using a combination of enzyme-linked immunosorbent assay and SDS-PAGE with protein blot (Western blotting), increased levels of serum platelet bindable immunoglobulin (SPBIg) were demonstrated in 10 of 10 thrombocytopenic patients with systemic lupus erythematosus (SLE) (7.0-60 fg Ig/platelet) with consistent binding to SDS-PAGE platelet fractions of approximate molecular weight (120 and 80 kDa). This pattern of Ig binding was characteristic of SLE and was not seen in 20 normal volunteers and infrequently seen in 20 patients with idiopathic thrombocytopenic purpura.