A case of post-transfusion purpura in a 61-year-old, multiparous female with a platelet alloantibody (anti-Zwa) in her serum is reported. The patient was successfully treated with plasma exchange by means of a Haemonetics 30 cell separator and corticosteroids. Compared with other therapeutic measures, plasma exchange seems to shorten the duration of thrombocytopenia. Major surgery was possible in our patient within ten days of development of the syndrome.