Platelet destruction in chronic immune thrombocytopenic purpura (ITP) is due either to antibody against platelet-associated antigen(s) that attaches by the antigen-specific Fab portion of the molecule or to platelet-bound immune complexes that bind nonspecifically to a platelet Fc receptor. Since pepsin digestion destroys the Fc fragment, the effect of this agent on platelet binding should allow differentiation betwen these two mechanisms. Normal serum IgG, aggregated normal serum IgG, and IgG produced in culture by splenic cells from control subjects and ITP patients were radiolabeled and tested for platelet binding before and after pepsin digestion. The binding to target platelets of both aggregated IgG and IgG produced in culture by ITP cells was increased when compared to controls. However, F(ab)2 fragments from the ITP samples retained their binding ability while those from the aggregated IgG did not. We conclude that these ITP patients produced antibody specific for platelet-associated antigens.