Some in vitro studies on microglia suggest that microglial cells exhibit a toxic effect on neurons through the production of neurotoxic agents such as nitric oxide (NO) and superoxide anion (O 2 - ). However, there are several in vivo studies indicating the beneficial effects of microglial cells on neurons. To obtain some insights into this conflicting problem, we investigated the effects of microglial cells on neuronal survival in the presence of neurotoxic free radical donors including sodium nitroprusside (donor of NO), SIN-1 (NO and O 2 - ) and FeSO 4 (OH) that were added to primary cortical neuron cultures. These experiments were carried out under serum-free condition and in MEM containing 25 μM serine, 5 μM glycine and 1 g/l glucose. Consequently, microglial cells protected neurons against free radical-induced cell death. We also compared the neuroprotective effect of microglial cells with that of astrocytes.