Leukoreduction of blood products is a technique used to prevent leukocyte-induced transfusion reactions. Filters currently used for human blood products achieve at least a 99.9% reduction in leukocyte numbers per unit (450 mL) of blood. Goals of this study were to determine if a prestorage leukoreduction filter could effectively achieve leukoreduction of canine blood and to determine if viability of the leukoreduced red blood cell (RBC) product could be maintained after 35 days of storage. Blood collected from each dog was filtered through a leukoreduction filter at either room temperature or after cooling (4 degrees C) for 4 hours. Filtration efficacy was determined by measurement of pre- and postfiltration leukocyte counts. In vitro viability of RBCs was determined by comparing RBC adenosine triphosphate concentration and percent hemolysis before and after the storage period. In vivo viability of stored cells was determined using a biotin-streptavidin-phycoerythrin labeling technique and flow cytometry. Blood filtered within 30 minutes of collection versus blood filtered after cooling had mean reductions in leukocyte numbers of 88.90 and 99.99%, respectively. The mean ATP and hemoglobin concentrations from the in vitro analysis were comparable to those obtained in previously for canine RBC adequately stored for 35 days. The mean in vivo 24-hour survival of the stored RBC was 84.7%. The leukoreduction filter used did not adversely affect in vitro or in vivo viability of canine RBCs. The filter effectively removed leukocytes from blood, with maximal efficiency of filtration achieved with use of cooled blood.