During a 7-month period, 29 units of feline whole blood in a hospital blood bank were confirmed, and 2 units were suspected, to be contaminated with Serratia marcescens. An investigation of the outbreak identified S marcescens in a jar of alcohol-soaked cotton balls and in a bag of saline solution used during venipuncture. Fifteen of the contaminated units were administered to 14 cats, and 6 of the 14 developed clinical signs of a transfusion reaction. The most common sign was vomiting; 4 cats died. The report underscores the importance of using aseptic techniques during collection of blood for transfusion and of thoroughly investigating any transfusion reaction.