A case of posttransfusion purpura is reported in a 90-year-old patient whose PlA1 antibody (anti-HPA-1a) was found to bind better to HPA-1a in the presence of captopril, a drug the patient had taken. Initially, IgG antibodies were found in the serum that reacted with normal platelets, but the binding of the antibody was increased in vitro by captopril, which suggested that captopril was responsible for the thrombocytopenia. However, in vitro studies demonstrated that the patient's platelets were negative for HPA-1a and that anti-HPA-1a was present in the serum, both of which findings were consistent with the diagnosis of posttransfusion purpura. The binding of this antibody was enhanced 50 percent by captopril in vitro, and increased binding in the presence of captopril did not occur when the anti-HPA-1a was removed. Similar results were obtained with serum containing anti-HPA-1a from another patient with posttransfusion purpura. Thus, captopril may increase the binding of anti-HPA-1a and confuse the determination of the cause of acute thrombocytopenia.