Fulminant viral hepatitis (FH) remains an important clinical problem in which the underlying pathogenesis is not well understood. Here, we present insight into the immunological mechanisms involved in FH caused by murine hepatitis virus strain 3 (MHV-3), indicating a critical role for CD4+CD25+ regulatory T cells (Tregs) and production of the novel Treg effector molecule FGL2. Before infection with MHV-3, susceptible BALB/cJ mice had increased numbers of Tregs and expression of fgl2 messenger RNA (mRNA) and FGL2 protein compared with resistant A/J mice. After MHV-3 infection, plasma levels of FGL2 in BALB /cJ mice were significantly increased, correlating with increased percentage of Tregs. Treatment with anti-FGL2 antibody completely inhibited Treg activity and protected susceptible BALB/cJ mice against MHV-3-liver injury and mortality. Adoptive transfer of wild-type Tregs into resistant fgl2-/- mice increased their mortality caused by MHV-3 infection, whereas transfer of peritoneal exudate macrophages had no adverse effect. Conclusion: This study demonstrates that FGL2 is an important effector cytokine of Tregs that contributes to susceptibility to MHV-3 - induced FH. The results further suggest that targeting FGL2 may lead to the development of novel treatment approaches for acute viral hepatitis infection.