The effects of ccpharanthin and cytochalasin D on the internalization of anti-glycoprotein Ilb/llla antibodies by platelets were investigated in 13 patients with chronic immune thrombocytopenic purpura who had circulating anti-glycoprotcin Ilb/IIIa autoantibodies. Unfixed platelets were incubated with a monoclonal anti-glycoprotcin Ilb/IIIa antibody (NNKY1-32) or with platelet-binding IgG from the patients (which contained anti-glycoprotein Ilb/IIIa antibodies). Flow cytometry showed that the binding of NNKY 1-32 to platelets was markedly decreased after incubation for 120 min compared with incubation for 10 min. This decrease was inhibited by ccpharanthin but not by cytochalasin D. Platelet-binding IgG also showed markedly reduced binding after incubation for 120 min compared with 10 min, and this decrease was inhibited by both ccpharanthin and cytochalasin D. Cytochalasin D inhibits platelet cytoskcletal activity while cepharanthin does not. Therefore, our results suggest that the internalization of anti-glycoprotcin Ilb/IIIa antibodies from the plasma of patients with immune thrombocytopenic purpura is related to platelet cytoskeletal reorganization, while the cytoskelton did not participate in internalization of the monoclonal anti-glycoprotcin Ilb/IIIa antibody (NNKY1-32). Cepharanthin may be useful for studying the internalization and cycling of glycoprotein Ilb/IIIa in human platelets, and it may also be potentially useful for the treatment of immune thrombocytopenic purpura.