AbstractPost‐transfusion purpura (PTP) with severe thrombocytopenia occurred eight days after transfusion in a 28‐year‐old woman and responded to treatment with prednisone and plasma exchange. In contrast to nearly all previously studied cases of PTP, the patient's platelets were PIAI‐positive and anti‐PIAIantibody could not be detected in serum obtained during the thrombocytopenic episode. Her serum was found to contain an antibody specific for a recently described platelet‐specific alloantigen, Baka, in addition to multiple HLA‐specific antibodies. The patient's platelets, typed following recovery, were Baka‐negative. These findings indicate that post‐transfusion purpura can occur in association with alloimmunization to platelet‐specific antigens other than PIAI.In performing the serologic studies, a close relationship and possible identity between Bakaand another recently reported platelet antigen, Leka, was observed. A method for analyzing mixtures of cytotoxic platelet‐reactive antibodies without separating the individual an