A patient with acute leukemia who typed as AintB received 10 packs of platelets of group 0. Subsequent transfusion of A1B blood resulted in a hemolytic transfusion reaction. Anti‐A1was detected in the serum and on the red blood cells of a post‐transfusion sample. This anti‐A1reacted with the transfused A1B red blood cells and other A1cells, but not with the patient's pretransfusion red blood cells. The plasma of the transfused platelet concentrates had a high titer of immune a