AbstractActivated microglia may contribute to two opposite effects during inflammation within the central nervous system: host defense against microorganisms and neuronal injury. Each of these processes may be mediated by the generation of reactive oxygen intermediates by activated microglia. We investigated the effects of two proinflammatory cytokines, interferon (IFN)‐β and tumor necrosis factor (TNF)‐α, and of the anti‐inflammatory cytokine, transforming growth factor (TGF)‐β, on murine microglial cell superoxide (O2) production upon stimulation with phorbol myristate acetate (PMA). Priming of microglia with IFN‐β or TNF‐α resulted in a dose‐dependent enhancement of O2release in response to PMA. The priming effects of these two cytokines were additive, suggesting that they acted by independent mechanisms. We also found that IFN‐β and TNF‐β stimulated the release of bioactive TGF‐β and that treatment of microglial cell cultures with TGF‐β antagonized the priming effects of IFN‐β and TNF‐α on O2production. The results of this study have implications for understanding the mechanisms by which cytokines and microglia may contribute to host defense as well as to injury