Little is known about the role of granulocyte colony-stimulating factor (G-CSF) in the response to chronic bacterial infections. To address this we infected G-CSF knock out (G-CSF(-/-)) mice with Mycobacterium avium. Infection was not exacerbated in G-CSF(-/-) mice despite a deficiency in the total bone marrow cells, colony-forming haemopoietic cells, granulocytes and monocyte precursors in the bone marrow. Peritoneal cells from G-CSF(-/-) produced less nitric oxide (NO) upon culture in vitro with antigen than did wild-type (WT) cells. Unexpectedly, T cells from infected G-CSF(-/-) mice were able to produce significantly more interferon-γ (IFN-γ) than the wild type (WT) controls. T cells from G-CSF(-/-) mice still produced more IFN-γ even when in vitro NO production was inhibited, while enzyme-linked immunospot assay (ELISPOT) assays showed more IFN-γ-producing cells in the G-CSF(-/-) mice. This was confirmed by intracellular cytokine staining (ICCS), which showed that there were more IFN-γ producing T cells in vivo in the G-CSF(-/-) than the WT controls following M. avium infection. It is possible that a deficit of NO in vivo allows T cells to develop a higher IFN-γ-producing phenotype. Thus we show a novel relationship between G-CSF and IFN-γ production by T cells revealed in this chronic bacterial infection model.