Microglia are the primary immune effector cells resident within the CNS, whose activation into migratory, phagocytic cells is associated with increased expression of cell adhesion molecules of the integrin family. To determine which specific factors are important regulators of microglial activation and integrin expression, we have examined the influence of individual cytokines and extracellular matrix (ECM) substrates by quantifying cell surface expression of MHC and individual integrins by flow cytometry. We found that the proinflammatory cytokines TNF and IFN-α promoted microglial activation, as assessed by amoeboid morphology and increased expression of MHC class I, and also increased expression of the α 4 β 1 and Mac-1 integrins. In contrast, TGF-β1 had the opposite effect and was dominant over the other cytokines. Furthermore, the ECM substrates fibronectin and vitronectin, but not laminin, also promoted microglial activation and increased expression of the α 4 β 1, α 5 β 1 and Mac-1 integrins, but significantly, the influence of fibronectin and vitronectin was not diminished by TGF-β1. Taken together, this work suggests that, in addition to cytokines, the ECM represents an important regulatory influence on microglial activity. Specifically, it implies that increases in the local availability of fibronectin or vitronectin, as a result of blood-brain barrier breakdown or increased expression in different pathological states of the CNS, could induce microglial activation and increased expression of integrins.