In a prospective investigation on the transfusion of blood to 209 patients,a distinct difference was found between the frequency of leuko-agglutinins and lymphocytotoxic antibodies in the sera of patients with reported febrile transfusion reactions and patients without. No such difference, however, was demonstrated concerning thrombocyte agglutinins and thrombocyte complement-fixing antibodies. A significant difference was found between the incidence of transfusion reactions in patients with leukocyte antibodies in their sera and in patients without. If sera from patients with reported transfusion reactions were matched against leukocytes and trombocytes of the respective donors, incompatibility was demonstrated in all cases. Minor incompatibility was found to be a most unlikely cause of transfusion reactions in this material. The results of our study are thus confirming the concept that leukocyte incompatibility is causually related to febrile transfusion reactions, whereas evidence concerning the influence of thrombocyte incompatibility still is too small to be conclusive.