A new approach to the study of antiplatelet antibodies is described. Purified IgG from sera or from the culture of splenic ells was radiolabeled with 125I. Incubation of radiolabeled splenic IgG from ITP patients showed significantly greater binding to target platelets when compared to control IgG samples; conversely, the binding to target platelets of 125I-labeled serum IgG from ITP patients did not differ from controls. In each of the six instances when the binding of splenic and serum IgG from the same ITP patients was compared, the splenic samples contained a much higher concentration of platelet-specific IgG. This suggests that intrasplenic platelets in ITP patients are exposed in vivo to high concentrations of antiplatelet antibody and may explain, in part, the importance of the spleen in platelet destruction in this disease. This assay should prove useful in the further study of immune thrombocytopenia.