The development of effective supportive therapy for myelosuppression could be expected to protect patients with hematological malignant disease against morbidity and mortality and to expand the potential of therapeutic agents. Although the technique of plateletpheresis has largely solved the problem of obtaining enough platelets from donors, available techniques for short and long term platelet preservation are inadequate. Refractoriness to platelet transfusions has been shown to result from development of antibodies to the HL-A system, and by HL-A matching of platelet donors and recipients it may be possible to reduce the numbers of units of platelets necessary to maintain thrombocytopenic patients and thereby to decrease the incidence of isosensitization and transfusion hepatitis. Both the NCI-IBM blood cell separator and the reversible adhesion of leukocytes to nylon fibers have been used to obtain sufficient quantities of granulocytes from normal donors for transfusion to neutropenic patients. Patients with chronic myelogenous leukemia still represent an important source of granulocytes for transfusion, and we have found that repeated leukapheresis of such patients produced good symptomatic improvement. Granulocyte transfusions have been effective in the treatment of infections in patients with acute leukemia. In our experience more of the administered granulocytes have been recovered in the peripheral blood of recipients when the donor and the recipient have been identical twins, but the clinical response to granulocyte transfusions has not been related to compatability of the HL-A or ABO systems or to the presence or absence of leukoagglutinins. © 1974.