Ticlopidine is a new drug for the inhibition of the aggregation of platelets, which is used in arteriosclerosis. Here we describe a patient, who developed a thrombocytopenic purpura whilst using this drug. After stopping the drug, platelet counts normalized in two weeks. In the serum of this patient antibodies were found which reacted with the Ticlopidine exposed platelets of the patient after recovery and with those of healthy donors. These antibodies did not react when these platelets were exposed to tetrahydro-4,5,6,7,thiéno(3,2-c)pyridine, a metabolite of Ticlopidine. We conclude that after adherence of the drug to the platelet membrane, antibodies induced by Ticlopidine destroy the platelets as carriers of the drug.