Microglial cells constitute what is considered to be a fixed macrophage population in the central nervous system (CNS), which are broadly implicated in the regulation of neuroinflammation. In the normal adult CNS, microglial cells exist in a resting state characterized by a minimal or negative expression of MHC class II and the co-stimulatory molecules CD80, CD86 and CD40 and exhibit a unique ramified morphology. Microglial cell activation is associated with many inflammatory and neurogenerative CNS pathologies and is characterized by the transformation of resting microglia into cells with a macrophage morphology and up-regulation of MHC class II and co-stimulatory molecules. The cellular and molecular mechanisms required for microglial cell activation and their immunological functions in the adult brain still remain enigmatic, primarily due to the lack of an appropriate culture system that both facilitates microglial survival and expansion in the resting state. Here, we describe a new M-CSF-dependent culture system that overcomes these barriers and allows the long-term proliferation and maintenance of resting adult microglial cells isolated from the CNS. These cultured microglial cells retain their plasticity as indicated by their ability to up-regulate MHC class II and differentiate into cells with a macrophage morphology following the addition of IFN-gamma and GM-CSF, or activated T cells, which produce both cytokines. By measuring the proliferation of the T cells, we were also able to demonstrate that the microglial cells differentiated into fully functional antigen presenting cells. In addition, the replacement of the M-CSF with GM-CSF resulted in the differentiation of microglial cells into cells morphologically and phenotypically similar to dendritic cells. Our microglial cell culture system is the first described that allows the expansion of adult cells in the resting state and will facilitate studies examining the specific mechanisms of microglial cell activation and functions involved in a variety of CNS pathologies.