Stimulation of mouse astrocytes and microglia in enriched culture by lipopolysaccharide (LPS) results in the production of a cytotoxic factor that is functionally similar to macrophage-derived tumor necrosis factor-alpha (TNF). This cytotoxic factor kills murine L929 cells, and is neutralized by a monoclonal anti-mouse TNF. The production of this factor is inhibited by pretreatment of cells by actinomycin D. Microglia are more sensitive to LPS and produce much higher activity of cytotoxicity in lower concentration of LPS than astrocytes. These results suggest that microglia are the major producers of TNF in the brain and may play a role in certain pathological conditions in the brain.