Chronic immune thrombocytopenic purpura (ITP) is an autoimmune disorder due to autoantibody-induced destruction of platelets. Several forms of therapy have been used, including corticosteroids, splenectomy, danazol, and a variety of immunosuppressants. We studied the mechanism of action of some of these treatments by evaluating patients' platelet- associated autoantibodies (PAAb) and platelet count before and serially following therapy. Treatment with corticosteroids, splenectomy, cyclophosphamide, and combination chemotherapy resulted in a progressive decrease in PAAb associated with an improvement in the platelet count, and appeared to act by primarily affecting autoantibody production. Conversely, PAAb levels either remained stable or increased during vincristine or danazol therapy despite improvement in the platelet count, suggesting that the major effect of these agents was decreased platelet removal by the reticuloendothelial (RE) system.