Objective:To study possible immunobiological potential of Osmunda japonica Thunb.Methods:Immunomodulatory effects of ethanol extracts prepared from rhizomes of O.japonica and phenolic compounds isolated from the extracts were investigated under the In vitro conditions using the rat peritoneal cells(2×10~6/mL;24 h culture).Biosynthesis of nitric oxide(NO) was assayed by Griess reagent,production of prostaglandin E_2(PGE_2) and secretion of cytokines were determined by enzyme-linked immunoabsorbent assay.Results:The extracts activated dose dependently,with the onset at 2.5-5 μmol/L concentrations,the high output NO production,and secretion of interferon-γ(IFN-γ),tumor necrosis factor-α(TNF-α) and interleukin-1 β(IL-1 β).Mild enhancement of NO was produced by the aldehyde-type phenolics 4-hydroxybenzaldehyde and 3,4-hydroxybenzaldehyde.In contrasts,the acetone-type phenolics 4-hydroxybenzalacetone and 3,4-hydroxybenzalacetone inhibited production of immune mediators including cytokines(TNF-α,IL-1 β,IL-6),NO,and PGE_2.The 3,4-hydroxybenzalacetone was more effective than 4-hydroxybenzaldehyde.The IC_(50)s estimates ranged within the interval of 5-10 μmol/L.No signs of cytotoxicity were observed up to the 50 μmol/L concentration of the compounds.Conclusion:Phenolic compounds contained in medicinal herb Osmunda japonica possess distinct immunomodulatory activity. Objective: To study possible immunobiological potential of Osmunda japonica Thunb. Methods: Immunomodulatory effects of ethanol extracts prepared from rhizomes of O. japonica and phenolic compounds isolated from the extracts were investigated under the in vitro conditions using the rat peritoneal cells (2 × 10 ~ 6 / mL; 24 h culture). Biosynthesis of nitric oxide (NO) was assayed by Griess reagent, production of prostaglandin E_2 (PGE_2) and secretion of cytokines were determined by enzyme-linked immunoabsorbent assay. Results: The extracts activated dose dependently, with the onset at 2.5-5 μmol / L concentrations, the high output NO production, and secretion of interferon-γ (IFN-γ), tumor necrosis factor-α (TNF-α) and interleukin- .Mild enhancement of NO was produced by the aldehyde-type phenolics 4-hydroxybenzaldehyde and 3,4-hydroxybenzaldehyde. In contrasts, the acetone-type phenolics 4-hydroxybenzalacetone and 3,4-hydroxybenzalacetone inhibited production of immune mediators including cyto kines (TNF-α, IL-1β, IL-6), NO, and PGE_2.The 3,4-hydroxybenzalacetone was more effective than 4-hydroxybenzaldehyde.The IC 50 estimates ranged within the interval of 5-10 μmol / L. No signs of cytotoxicity were observed up to the 50 μmol / L concentration of the compounds. Confclusion: Phenolic compounds contained in medicinal herb Osmunda japonica possess distinct immunomodulatory activity.