The role of microglia in host defense against Toxoplasma gondii is unknown. In the present study, we investigated the multiplication of T. gondii tachyzoites in murine microglial cell cultures. T. gondii multiplied readily in these cells; multiplication was prevented when microglia were activated with interferon-γ plus lipopolysaccharide, a treatment that also upregulates nitric oxide (NO) synthase activity. Simultaneous treatment of microglial cell cultures with activation signals and the NO synthase inhibitor NG-monomethyl-Larginine (NGMA) prevented the antitoxoplasmic activity. Transmission electron microscopic analysis demonstrated degenerative tachyzoites in activated microgloa but not in control or NGMA groups. These findings support the view that the host defense function of activated microglia against T. gondii involves generation of the free radical NO.