Glutamate excitotoxicity is implicated in the progressive loss of oligodendrocytes in multiple sclerosis, but how glutamate metabolism is dysregulated in the disease remains unclear. Because there is microglia activation in all stages of multiple sclerosis, we determined whether a microglia product, interleukin-1beta, could provide the mechanism for glutamate excitotoxicity. We found that whereas interleukin-1beta did not kill oligodendrocytes in pure culture, it produced apoptosis of oligodendrocytes in coculture with astrocytes and microglia. This requirement for a mixed glia environment suggests that interleukin-1beta impairs the well-described glutamate-buffering capacity of astrocytes. In support, antagonists at AMPA/kainate glutamate receptors, NBQX and CNQX, blocked the interleukin-1beta toxicity to oligodendrocytes. Another microglia/macrophage cytokine, tumor necrosis factor-alpha, also evoked apoptosis of oligodendrocytes in a mixed glia environment in an NBQX-blockable manner. These results provide a mechanistic link between the persistent and insidious microglia activation that is evident in all stages of multiple sclerosis, with the recent appreciation that glutamate excitotoxicity leads to the destruction of oligodendrocytes in the disease.