A patient with the rare syndrome of simultaneous quinidine[an antiarrhythmic]-induced thrombocytopenia and leukopenia was studied. A quinidine-dependent antiplatelet antibody was detected in her serum by platelet aggregometry and by indirect platelet-suspension immunofluorescence. A drug-dependent antileukocyte antibody was demonstrated by leukoagglutination and by granulocyte immunofluorescence. Both antibodies were found to belong to the IgG class by immunofluorescence using monospecific antisera. There appeared to be 2 distinct antibodies, since the antibody eluted off sensitized platelets reacted only with platelets and that eluted off sensitized granulocytes reacted only with granulocytes. The patient''s serum and quinidine gave a negative reaction on immunofluorescence with platelets from a patient with Bernard-Soulier syndrome (BSS), which lack glycoprotein Ib (GPIb) complex, but gave a positive reaction with BSS granulocytes. The quinidine-dependent antiplatelet antibody immunoprecipitated GPIb and a glycoprotein of MW 22,000 (GPIb complex) from Triton-solubilized, periodate-labeled platelets. Similar attempts to identify the granulocytes surface receptor/antigen were unsuccessful. Factor VIII/von Willebrand factor was not required for binding of the drug-dependent antibody to platelets.