The 80% aqueous acetone extract of the rhizomes of Alpinia galanga was found to inhibit release of beta-hexosaminidase, as a marker of antigen-IgE-mediated degranulation in RBL-2H3 cells. Nine known phenylpropanoids and p-hydroxybenzaldehyde were isolated from the extract. Among them, 1'S-1'-acetoxychavicol acetate and 1'S-1'-acetoxyeugenol acetate exhibited potent inhibitory activity with IC(50) values of 15 and 19 microM. From the effects of various related compounds, both the 1'- and 4-acetoxyl groups of 1'S-1'-acetoxychavicol acetate and 1'S-1'-acetoxyeugenol acetate were essential for their strong activity, and the 2'-3' double bond enhanced the activity. In addition, 1'S-1'-acetoxychavicol acetate and 1'S-1'-acetoxyeugenol acetate inhibited ear passive cutaneous anaphylaxis reactions in mice and the antigen-IgE-mediated TNF-alpha and IL-4 production, both of which participate in the late phase of type I allergic reactions, in RBL-2H3 cells.