The mechanism of drug-dependent immunologic thrombocytopenic purpura (DITP) was investigated by studying the sera of four patients with classic DITP (two with quinidine-, one with acetaminophen-, and one with phenazopyridine-dependent antiplatelet antibody) using a solid- phase radioimmunoassay with 125I-staphylococcal protein A. Two forms of antiplatelet antibody could be demonstrated: one that required drug to bind to platelets and one that bound to platelets in the absence of drug. Drug-dependent antiplatelet antibody required the simultaneous addition of drug and the Fc domain of the drug-dependent IgG molecule for binding to platelets. It did not require serum complement or factor VIII-related antigen for binding to platelets. Drug-dependent binding of antibody to platelets was saturation-dependent. Non-drug-dependent antiplatelet antibody of two patients (one with quinidine-induced thrombocytopenia and the other with acetaminophen-induced thrombocytopenia) reacted with autologous platelets as well as with homologous platelets, indicating that they were autoantibodies. Both autoantibodies had disappeared when their sera were tested 23 and 138 days, respectively, after withdrawal of their initial positive sera. Non-drug-dependent antiplatelet antibody binding could be demonstrated with the F(ab')2 fragment of the purified IgG of the serum of the second patient with quinidine DITP, who did not have detectable alloantibodies against HLA. None of the four patients with non-drug- dependent antiplatelet antibody had a past or present history of autoimmune thrombocytopenic purpura.