In this report, we show that the human astroglioma cell line, D54-MG, constitutively expresses C3 mRNA and secretes antigenically detectable C3 protein. the cytokine interferon-gamma (IFN-γ) enhances C3 mRNA and protein expression by D54-MG cells in a dose- and time-dependent manner. C3 mRNA from both D54-MG cells and primary human adult astrocytes has the same apparent size (5.1-5.2 kb) as C3 mRNA from hepatocyte and monocyte cell lines. Constitutive C3 mRNA levels in D54-MG cells and primary human astrocytes are comparable. Primary rat astrocytes also constitutively express C3 mRNA, which is enhanced upon exposure to IFN-γ. These data are novel since expression of C3 in other cell types is refractory to IFN-γ. In the central nervous system (CNS), endogenous complement production by astrocytes, and enhancement by the cytokine IFN-γ, may contribute to the pathogenesis of inflammatory demyelinating diseases such as multiple sclerosis (MS). © 1992.