The effect of aqueous extract of Isodon japonicus Hara (Labiatae) (IJAE) on mast cell-mediated immediate-type allergic reactions was investigated. IJAE inhibited compound 48/80-induced systemic anaphylaxis and immunoglobulin E (IgE)-mediated local anaphylaxis. When IJAE was pretreated at the same concentration with systemic anaphylaxis, serum histamine levels were reduced in a dose-dependent manner. IJAE dose-dependently inhibited histamine release from rat peritoneal mast cells (RPMC) activated by compound 48/80. The level of cAMP in human mast cell line (HMC-1) cells, when IJAE was added, significantly was increased, compared with that of normal control. These results indicate that IJAE will beneficial in the treatment of immediate-type allergic reaction.