A 38-year-old woman (JT) was diagnosed with posttransfusion purpura and significant posthysterectomy vaginal bleeding 9 days after the transfusion of 2 U of packed red blood cells. Analysis of JT's serum by a monoclonal antibody-antigen capture enzyme-linked immunosorbent assay method showed the presence of anti-HPA-5b (anti-Bra) antibodies directed against an epitope on platelet glycoprotein (GP) la of the GPIa/IIa complex. The patient's serum immunoprecipitated two proteins from 125I-labeled HPA-5b positive platelets that migrated under both nonreducing and reducing conditions on sodium dodecyl sulfate polyacrylamide gels at molecular weights characteristic of GPIa (150 Kd and 165 Kd, respectively) and GPIIa (120 Kd and 145 Kd, respectively). These bands were not precipitated when 125I-labeled HPA-5b negative platelets were used. Platelet typings performed on JT and her three children showed that the patient was HPA-5b negative and one of her children was HPA-5b positive. Platelets obtained from one of the donors who provided blood for the inciting transfusion also typed as HPA-5b positive. These findings demonstrate that posttransfusion purpura may be induced by antibodies directed against an alloantigenic epitope, namely HPA-5b (Bra), located on GPIa/IIa. Moreover, clinically significant bleeding can be associated with antibody reactions directed against this GP complex.