Colony stimulating factor-1 (CSF-1) regulates macrophage morphology and motility as well as mononuclear phagocytic cell proliferation and differentiation. The CSF-1 receptor (CSF-1R) transduces the pleiotropic signals of CSF-1 via autophosphorylation of up to 8 intracellular tyrosine residues. Using a novel bone marrow-derived macrophage cell line system to examine specific CSF-1R phosphotyrosine-activated signaling pathways, we demonstrate that macrophages expressing a Y721F mutant CSF-1R display striking morphological alterations. Signalling downstream of pY721 regulates adhesion and actin polymerisation to control macrophage spreading, motility and invasive capacity as well as their ability to enhance carcinoma cell invasion. The pY721 motif is the primary activator of CSF-1 induced macrophage motility via mediation of the direct association of PI3K with the CSF-1R. Work is now focussed on identifying specific inhibitors for this motility pathway to reduce macrophage infiltrative capacity and the promotion of tumour invasion and metastasis.