The transfusion of leukocyte-containing blood products can lead to the production of antibodies to antigens on the surface of leukocytes. Such antibodies can be detected by a variety of techniques including assays for lymphocytotoxicity, granulocytotoxicity, and leukoagglutination. In order to evaluate the effect of preformed anti-leukocyte antibodies during granulocyte transfusion therapy, recipient beagles were sensitized to donor foxhound antigens. After being made granulocytopenic with cyclophosphamide, these animals were transfused with a set dose of granulocytes collected by continuous flow centrifugation. When compared to the results of similar transfusions to nonsensitized recipients, granulocyte transfusions to animals with preformed anti-leukocyte antibodies resulted in lower one-hour posttransfusion leukocyte increments (p less than .04) and in less migration of neutrophils through a skin abrasion into a chamber containing a strong chemoattractant, autologous serum (p less than .0001). Also, profound thrombocytopenia was found in sensitized animals, but not in nonsensitized recipients, one hour after the granulocyte transfusion.