A patient with the rare syndrome of simultaneous quinidine-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 two distinct antibodies, since the antibody eluted off sensitized platelets reacted only with platelets and that eluted off sensitized granulocytes reacted only with granulocytes. In addition, the patientxy's serum and quinidine gave a negative reaction on immunofluorescence with platelets from a patient with Bernard-Soulier syndrome (BSS), which lack glycoprotein lb (GPlb) complex, but gave a positive reaction with BSS granulocytes. The quinidine-dependent antiplatelet antibody immunoprecipitated GPlb and a glycoprotein of molecular weight 22,000 (GPlb complex) from Triton- solubilized, periodate-labeled platelets. Similar attempts to identify the granulocyte surface receptor/antigen were unsuccessful. Factor VIII/von Willebrand factor was not required for binding of the drug- dependent antibody to platelets.