Although nearly all domestic shorthair and longhair cats have type-A blood (greater than 99%), the frequency of blood type B in various feline breeds ranges from 0 to 59%. All blood-type-B cats have strong natural alloantibodies, predominantly of the IgM class, whereas blood-type-A cats have low alloantibody titers of the IgG and IgM classes. We therefore studied the efficacy and safety of transfusing 20 ml of matched and mismatched 14C-potassium cyanate-labeled blood to cats. In autologous and allogeneic matched transfusions of blood-type-A and type-B cats, the half-life of labeled erythrocytes proved to be similar (29 to 39 days). In contrast, type-B erythrocytes transfused into 5 blood-type-A cats had a mean (+/- SD) half-life of only 2.1 +/- 0.2 days and induced minor transfusion reactions. Half of the type-A blood given to 4 blood-type-B cats was destroyed within minutes to 6 hours (mean +/- SD = 1.3 +/- 2.3 hours), depending on the alloantibody titer. After 1 day, none of the labeled erythrocytes were detected. Mismatched transfusions in blood-type-B cats caused marked transient reactions including systemic anaphylactic signs (hypotension, bradycardia, apnea, urination, defecation, vomiting, and severe neurologic depression) and hemolytic signs (hemoglobinemia and pigmenturia) associated with severe reduction in plasma alloantibody titer and complement activity.(ABSTRACT TRUNCATED AT 250 WORDS)