We studied the action of methyleugenol on immediate hypersensitivity. Methyleugenol completely inhibited systemic anaphylaxis induced by compound 48/80 in mice. Methyleugenol al so inhibited local anaphylaxis induced by anti-dinitrophenyl (DNP) IgE. Moreover, methyleugenol dose-dependently inhibited histamine release in peritoneal mast cells activated by compound 48/80 or anti-DNP IgE. These results suggest that the inhibitory effect of methyleugenol on anaphylaxis induced by compound 48/80 or anti-DNP IgE is due to, in part at least, the membrane stabilization of mast cells.