Allogeneic peripheral blood stem cell transplantation (PBSCT) is increasingly used instead of bone marrow transplantation, particularly in HLA identical sibling pairs. Despite the presence of significantly increased numbers of T cells in the PBSC graft, acute graft-versus-host disease (GVHD) is not increased. We have investigated whether granulocyte-colony stimulating factor (G-CSF) administration to PBSCT recipients, both with and without donor G-CSF pretreatment, further modulates acute GVHD in a murine model of PBSCT. Recipients of G-CSF mobilized splenocytes showed a significantly improved survival (P<0.001) and a reduction in GVHD score and serum LPS levels compared with control recipients. G-CSF treatment of donors, rather than recipients, had the most significant effect on reducing levels of tumor necrosis factor (TNFalpha) 7 days after transplantation. As a potential mechanism of the reduction in TNFalpha, we demonstrate G-CSF decreased dendritic cells TNFalpha, and interleukin-12 production to lipopolysaccharide. In conclusion, G-CSF modulates GVHD predominantly by its effects on donor cells, reducing the production of TNFalpha. G-CSF treatment of bone marrow transplantation recipients, without pretreatment of the donor, does not have an impact on acute GVHD.