Granulocyte colony-stimulating factor (G-CSF) is a haemopoietic cytokine first identified for its role in steady state granulopoiesis but is now also recognised as a proinflammatory mediator of mature neutrophil activation and function. G-CSF binds to the G-CSF receptor (G-CSFR), which is present on neutrophils and haemopoietic progenitors. G-CSF-deficient mice are protected from disease in several models of rheumatoid arthritis, and blockade of G-CSF is also protective in those models. To further investigate the actions of blocking G-CSF/G-CSFR signalling in inflammatory disease, we developed a neutralising monoclonal antibody to mouse G-CSFR which prevents the binding of G-CSF and inhibits G-CSFR signalling. In the anti-collagen antibody induced arthritis model, anti-G-CSFR treatment rapidly halted the progression of established disease. This therapeutic effect was observed at low dose and was associated with transient suppression of peripheral blood neutrophilia. Anti-G-CSFR treatment significantly inhibited neutrophil accumulation in the joints without rendering animals neutropenic, suggesting an effect of G-CSFR blockade on neutrophil homing to inflammatory sites. Consistent with this, neutrophils in the blood and arthritic joints of anti-G-CSFR treated mice showed alterations in homing receptors, with reduced CXCR2 and increased CD62L expression. Microarray analysis of neutrophils from joints and peripheral blood showed a common up-regulation of 6 genes and down-regulation of 22 genes following anti-G-CSFR treatment. Blocking neutrophil traffic to joints with anti-G-CSFR suppressed local production of proinflammatory cytokines (IL-1b and IL-6) and chemokines (KC and MCP-1) known to drive tissue damage. These data show for the first time the effect of blockade of G-CSFR in a therapeutic model of inflammation and provide further support for utilising this approach in the treatment of inflammatory diseases such as rheumatoid arthritis.