A new cyclic diarylheptanoid, acerogenin M (1), has been isolated along with nine known diarylheptanoids, 2-10, and two known phenolic compounds, 11 and 12, from a MeOH extract of the stem bark of Acer nikoense MAXIM. (Aceraceae). The structure of 1 was determined on the basis of a spectroscopic method. Upon evaluation of the inhibitory effects on 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced inflammation (1 microg/ear) in mice of nine of the compounds (2-6, 8, 10-12), six (2, 4-6, 8, 10) showed a marked anti-inflammatory effect with a 50% inhibitory dose (ID(50)) of 0.26-0.81 mg per ear. In addition, upon an evaluation against the Epstein-Barr virus early antigen (EBV-EA) activation induced by TPA for all of the compounds, all exhibited moderate inhibitory effects against EBV-EA induction (IC(50) values of 356-534 mol ratio/32 pmol TPA).