To the Editor: Post-transfusion purpura is an acute episode of immune thrombocytopenia that follows the administration of certain blood products by seven to ten days. We have previously described the diagnostic use of the platelet aggregometer in this disorder and the rapid therapeutic efficacy of plasmapheresis (Abramson N, Eisenberg PD, Aster RH: Post-transfusion purpura: immunologic aspects and therapy. N Engl J Med 291:1163, 1974). The following patient studies reemphasize these points:A 51-year-old woman received 4 units of whole blood during a spinal fusion. Twelve days later she vomited blood, and the hemoglobin fell 3 g per deciliter. Coagulation . . .