Activation of local tissue macrophages (Kupffer cells) and of quiescent hepatic stellate cells (HSCs) to a myofibroblast phenotype are two key events in liver inflammation and fibrosis. It is known that products of activated macrophages may activate stellate cells. We have hypothesized that the products of activated HSCs may also modulate the activity of Kupffer cells. The cytokine interleukin-10 (IL-10), produced by lymphocytes and macrophages, has profound inhibitory actions on macrophages. Normal rat and mouse HSCs that differentiate in vivo and in vitro to activated myofibroblasts were isolated by enzyme perfusion and density centrifugation with or without centrifugal elutriation, confirmed by vitamin A autofluorescence and positive immunostaining for the myofibroblast markers desmin and smooth muscle actin (SMA). Conditioned media and lysates from these cells were found to downregulate lipopolysaccharide (LPS)-induced tumor necrosis factor-α (TNF-α secretion by the mouse macrophage line RAW 267.4. In highly purified preparations of rat HSCs, messenger RNA (mRNA) for IL-10 was detected by reverse- transcription polymerase chain reaction (RT-PCR), from the time of isolation to up to 120 days of culture on plastic. Long-term cultures of unstimulated mouse HSCs secreted IL-10 protein as detected by immunoblotting and specific enzyme-linked immunosorbent assay (ELISA). IL- 10 protein was undetectable by immunohistochemistry in mouse HSCs for the first 3 days in culture. After this, the percentage of IL-10-positive cells increased to 45% at day 7 and 100% by day 14, and expression of IL-10 continued in long-term cultures of up to 120 days. The expression of IL-10 by the stromal cells that govern the fibrotic process in the liver may have important implications for the regulation of inflammation and fibrosis in the liver.