In this study, to investigate the effect on expression of IL-2 in lymphocytes from bone marrow and peripheral blood of normal donors after they were mobilized by G-CSF in allo-BMT, 7 normal donors bone marrow and peripheral blood were harvested before and after G-CSF administration. The separated lymphocytes were measured by FCM after they were stained intracellularly by anti-IL-2, and their expressions of IL-2 were compared. The degree of aGVHD in patients after bone marrow transplantation was evaluated clinically, and it was compared with the status of aGVHD of 15 patients whose donors didn't receive G-CSF administration in our department, and 2 groups of patients are comparable in age, types of diseases and status of donors. The results showed that the expression of IL-2 in lymphocytes in 7 G-CSF mobilized donors decreased significantly after G-CSF administration and more severe aGVHD than grade II didn't develop in these recipient patients, and comparing with 15 patients received the bone marrow from donors who didn't receive G-CSF, the incidence of aGVHD decreased. It is suggested that the expression of IL-2 in lymphocytes was influenced by donors' G-CSF administration, and it is likely that thereby reduces the incidence of aGVHD in patients after BMT.