Amyotrophic lateral sclerosis is a fatal, adult-onset motor neuron disease. A subset of cases is caused by mutations of superoxide dismutase 1 (SOD1) gene. The mechanisms how the mutations in this ubiquitous enzyme mediate the highly selective motor neuron degeneration, however, remain poorly understood. Recent results from transgenic animal models suggest a "non-cell autonomous" mechanism; i.e., cells other than neurons play an active role in motor neuron death. To investigate a possible effect of mtSOD1 on microglial cells, we compared primary cultured microglia from mtSOD1-transgenic mice and nontransgenic litter controls at neonatal (3 days) and adult (60 days) age. We found that mtSOD1 expression increases the production of TNF-alpha and attenuates IL-6-release by LPS-activated adult microglia. Neonatal microglia, however, showed no differences. Our findings suggest an increased cytotoxic potential of adult mtSOD1 microglia, which only becomes apparent after microglial activation.