The objective of this study was to investigate the actions of exogenous and endogenous IL‐10 on inflammatory responses of glia. Studies were conducted in primary, mixed glial cultures from C57BL/6 (wild‐type [WT]) and IL‐10‐deficient C57BL/6 (IL‐10 knockout [KO]) neonatal mice. Activation of cultures from WT mice by bacterial lipopolysaccharide (LPS, 10 ng/ml–10 μg/ml, 24 h), caused dose‐dependent increases in nitric oxide (NO) and prostaglandin E₂ (PGE₂) release. In cultures from IL‐10 KO mice, LPS elicited markedly attenuated release of NO (approximately 4‐fold) and PGE₂ (approximately 17‐fold). In WT cultures, co‐incubation with IL‐10 (10 or 100 ng/ml, 24 h) inhibited the effects of LPS on release of NO (30%) and PGE₂ (40–50%). In cultures from IL‐10 KO mice, the addition of IL‐10 (10 or 100 ng/ml, 24 h) completely abolished LPS‐induced NO and PGE₂ release. LPS did, however, release of IL‐1β and TNF‐α in cultures from all animals. Co‐incubation of WT cultures with IL‐10 (1, 10, or 100 ng/ml, 24 h) dose‐dependently reduced the release of IL‐1β (by 0%, 15%, 75%, respectively). In cultures from IL‐10 KO mice, co‐incubation with IL‐10 (1, 10, or 100 ng/ml, 24 h) completely abolished LPS induced release of IL‐1β. Co‐incubation with IL‐10 (1, 10, 100 ng/ml) reduced, LPS‐induced TNF‐α release dose‐dependently in WT cultures (by 15%, 50% and 90%) and abolished LPS‐induced TNF‐α release in cells from IL‐10 KO mice. These results indicate that in glia from WT mice, exogenous IL‐10 attenuates LPS‐induces release of NO, PGE₂, TNF‐α and IL‐1β. In contrast, mixed glial cultures from IL‐10 KO mice showed reduced responses to LPS, but increased sensitivity to exogenous IL‐10. GLIA 33:97–106, 2001. © 2001 Wiley‐Liss, Inc.