Leukocytes were collected from donors with chronic myelocytic leukemia by plasmapheresis and transfused into severely leukopenic recipients. The median transfusion of 7 x 1010 granulocytes (range .15 to 35 x 1010) resulted in a median increase in circulating granulocytes of 1,000 per cu. nun.(range 0 to 19,OOO), one hour after injection. The posttransfusion increment was directly related to the number of cells injected. Only 4.8 per cent of the injected cells were recovered in the circulating blood volume at one hour (range 0-37 per cent). The per cent recovery was directly related to the pretransfusion granulocyte count of the recipient; the more severe the recipient's leukopenia, the lower w-as the per cent recovery of transfused cells. The transfused granulocytes disappeared from the recipients' circulation with a half time of 24 hours.A number of other factors were found to influence the results of transfusion, such as: the antileukemic drugs given, the presence of fever, the transfusion vehicle (k, saline concentrates or whole plasma), the ABO red cell compatibility of donor and recipient, and the sequence of transfusion.Clinical responses as measured by disappearance of fever were seen in 54 per cent of the recipients. The fraction of febrile patients responding increased as the dose of leukocytes transfused increased. Severe reactions, manifested by dyspnea, cyanosis, and lung infiltrates were seen in a small percentage, but febrile reactions occurred in 67 per cent of afebrile patients.