We studied the effect of aqueous extract of Magnolia officinalis bark (Magnoliaceae) (MOAE) on the immediate hypersensitivity reaction. MOAE (0.01 to 1 g/kg) dose-dependently inhibited compound 48/80 induced systemic anaphylaxis in rats. MOAE (0.1 and 1 g/kg) also significantly inhibited local immunoglobulin E (IgE)-mediated passive cutaneous anaphylactic reaction. When MOAE was pretreated at concentrations ranging from 0.01 to 1 g/kg, the levels of plasma histamine were reduced in a dose-dependent manner. MOAE (0.001 to 1 mg/ml) dose-dependently inhibited the histamine release from rat peritoneal mast cells (RPMC) activated by compound 48/80 or anti-dinitrophenyl (DNP) IgE. The level of cyclic AMP (cAMP) in RPMC, when MOAE was added, significantly increased compared with that of the normal control. Moreover, MOAE (0.01 to 1 mg/ml) had a significant inhibitory effect on anti-DNP IgE-induced tumor necrosis factor-alpha production from RPMC. These results indicate that MOAE inhibits immediate hypersensitivity reaction in vivo and in vitro.