Abstract.Remarkably high titers (80–160) of quinidine‐specific antibodies were detected in the serum of 5 patients with suspected quinidine purpura, when126I‐labeled staphylococcal protein A was used as the marker of IgG bound onto platelets. During a follow‐up of 3 of the patients, the antibody titers declined.The antibodies could be detected even when the final drug concentration was reduced from 0.3 to 0.003 mmol/I (i.e. therapeutic levels). The reaction between serum and platelets was independent of the type of quinidine salt used. No cross‐reactivity occurred with quinine.Heat inactivation of the serum did not decrease the quantity of specific antibodies detected on the platelets. The sedimentation behavior of antiplatelet activity in density gradient centri‐fugation resembled that of 7 S antibodies. When sedimentation was, however, effected on the drug‐serum mixture, the reactivity of