The transfusion of only 4 ml of group A blood resulted in sudden death of a cat that was recovering from surgery. The clinical signs were similar in many respects to those exhibited by tranquilized group B cats when challenge exposed for the 1st time with incompatible A cells. The distribution of the A and B blood group antigens and the occurrence of naturally occurring isoantibodies in the cat were considered in relation to the probability of producing transfusion reactions. The results obtained with tranquilized and anesthetized cats demonstrated that immediate reactions can occur following the 1st transfusion of incompatible red blood cells. Cross matching with respect to the AB system is recommended before giving blood transfusions to cats.