The cellular prion protein (PrPC) is a glycoprotein anchored by glycosylphosphatidylinositol (GPI) to the cell surface and is abundantly expressed in the central nervous system. Numerous studies have suggested a protective function for PrPC, including protection from ischemic and excitotoxic lesions and several apoptotic insults, and recent reports have shown that PrPC has a context-dependent neuroprotective function. In this study, we investigated the effect of PPNP down-regulation on various forms of microglial activation. We first examined the mRNA expression of PRNP upon exposure to IFN-γ, IL-4, or IL-10 in BV2 microglia. We then analyzed the effect of si-RNA-mediated disruption of PRNP on different parameters of microglial activation in IFN-γ-, IL-4-, or IL-10-stimulated microglia. The results showed that PRNP mRNA expression was invariably down-regulated in microglia upon exposure to IFN-γ, IL-4, or IL-10. PRNP silencing prior to cytokines treatment reduced the responsiveness of microglia to INF-γ treatment, significantly altered IL-4-induced microglial activation phenotype, and had no effect on IL-10-induced microglial activation. Together, these results support a role of PrPC in the modulation of the shift of microglia from a quiescent state to an activated phenotype and in the regulation of the microglial response during classical and alternative activation.