OPN (osteopontin) is a secreted glycoprotein predominantly expressed in bone matrix and kidney tissue. More recently, a neuroprotective role has been attributed to this cytokine since it can be up-regulated by microglia in neurodegeneration and inflammation. We demonstrate the expression of OPN within primary cultured microglia. Microglia incubated in vitro with different concentrations (0.1 fM–1 nM) of recombinant OPN showed increased proliferation at 10 fM. Moreover, conditioned medium of LLC-PK1 cells, a pig renal epithelial cell line and a known source of secreted OPN, more than doubled the rate of proliferation of microglia. Addition of an anti-OPN polyclonal antibody completely reversed this effect. Treatment with OPN dose-dependently also inhibited microglial superoxide production. In contrast, phagocytosis of fluorescent-labelled beads was enhanced by OPN. In conclusion, OPN shifts microglia, at least in vitro, to an alternative functional profile more fit to the immune-balanced microenvironment of the CNS (central nervous system).