Tests for leukoagglutinins and lymphocytotoxic antibodies were performed on sera from 34 recipients of 78 transfusions of leukocytes obtained from donors with chronic myelogenous leukemia. The presence of preformed leukocyte alloantibodies directed against donor cells predisposed to transfusion reactions of variable severity and failure of the transfused cells to survive and circulate in their new host. Studiesin vitrodemonstrated that these antibodies could inhibit granulocyte phagocytosis and bactericidal capacity. Pretransfusion compatibility testing for transfusion of leukocytes is recommended. Leukocytes should not be given if they are agglutinated by the recipient's serum or if the lymphocytes show damage in lymphocytotoxicity tests.