HLA alloimmunization and subsequent refractoriness to platelet transfusions are frequent complications of transfusion therapy in patients with hematologic malignancies. Contaminating leukocytes in red cell and platelet concentrates are responsible for primary HLA alloimmunization. In many clinical trials, it has been demonstrated that the use of leukocyte-poor blood products prepared by filtration could reduce HLA-alloimmunization from 40% to 15%. The immunogenic level cannot be determined accurately, but the previous studies show that a residual contamination below 1.10(6) leukocytes was efficient to prevent primary alloimmunization. Leukocyte depletion does not avoid secondary responses in patients having an history of pregnancies or transfusions. The UV-B irradiation of platelet concentrates, in combination with filtration, should further reduce alloimmunization in these presensitized patients.