Free-living amebae belonging to the genus Acanthamoeba are the causative agents of granulomatous amebic encephalitis, a chronic progressive disease of the central nervous system, and of amebic keratitis, a chronic eye infection. Granulomatous amebic encephalitis occurs more frequently in immunocompromised patients while keratitis occurs in healthy individuals. The recent increased incidence in Acanthamoeba infections is due in part to infection in patients with acquired immune deficiency syndrome, while that for keratitis is due to the increased use of contact lenses. Understanding the mechanism of host resistance to Acanthamoeba is essential since the amebae are resistant to many therapeutic agents. Studies in our laboratory as well as from others have demonstrated that macrophages from immunocompetent animals are important effector cells against Acanthamoeba. We have demonstrated also that microglial cells, resident macrophages of the brain, elicit cytokines in response to A. castellanii. Neonatal rat cortical microglia from Sprague-Dawley rats co-cultured with A. castellanii produced mRNA for the inflammatory cytokines, interleukin 1α, interleukin 1β, and tumor necrosis factor α. In addition, scanning and transmission electron microscopy revealed that microglia ingested and destroyed A. castellanii in vitro. These results implicate macrophages as playing an effector role against Acanthamoeba and suggest immune modulation as a potential alternative therapeutic mode of treatment for these infections.