This report describes an unusual hemolytic transfusion reaction. A group O donor was inadvertently transfused with 100 ml of group A red cells during the course of plasmapheresis. Granulocytopenia, thrombocytopenia, hypofibrinogenemia and low levels of factors V, VIII, and IX developed immediately after the infusion. The levels of the cellular elements and coagulation proteins returned to normal in less than an hour; too rapid for ade novosynthesis after consumption of coagulation proteins. We postulate that activation of the kinin system was responsible for the phenomena rather than intravascular coagulation with secondary fibrinolysis.