51 leukocyte transfusions from healthy donors and 3 from chronic myelogenous leukemia were given to 16 patients with acute leukemia and 4 with aplasia. During 14 transfusions we have observed clinicals reactions which are of immunological origin. The part of the transfusions and of the pregnancies in the presence of antibodies is argued. The necessity of utilisation of HL-A compatible donors during leukocyte transfusions is asserted.