Serum soluble interleukin-2 receptor - alpha (sIL-2R) levels markedly increased at the engraftment period in patients who underwent allogeneic bone marrow transplantation (BMT). Since serum G-CSF levels increased during G-CSF administration and decreased after the cessation, increased sIL-2R levels appeared to be induced by G-CSF administration. There was no increase in sIL-2R levels in a patient given macrophage colony-stimulating factor (M-CSF). The sIL-2R levels at the engraftment period and the onset of acute graft-versus-host disease (GVHD) were higher in patients who developed acute GVHD during G-CSF administration than in those who developed acute GVHD after G-CSF cessation. This finding suggests that G-CSF administration may possibly augment acute GVHD. However, it appears to be unlikely, because in the entire population, 18 of 35 patients had acute GVHD while only 6 of 17 patients had acute GVHD during G-CSF administration. Further analysis is still needed in order to draw definite conclusions. Preconditioning regimens did not appear to affect the sIL-2R levels, when the variable frequencies of methotrexate (MTX) administration were compared.