A case of posttransfusion purpura was reported in which the laboratory determination of the specificity of the causative antibody was initially confused by the presence in the patient''s serum of HLA-directed antibodies. The patient was a multiparous 65-yr-old woman with a previous history of blood transfusion. She developed the typical clinical features of posttransfusion purpura 8 days following the transfusion of 6 units of packed red cells. The patient was P1A1 negative and her serum reacted strongly in a radiolabeled monoclonal antibody assay with both P1A1-positive and -negative platelets. Radioimmunoprecipitation demonstrated that the patient''s serum contained antibodies both of anti-P1A1 and anti-HLA specificity. Western blotting of normal platelets incubated with the patient''s serum verified that the anti-P1A1 antibody was directed against platelet membrane glycoprotein III.