The effects of eugenol, a major component of clove, on anaphylaxis were evaluated in rats. Eugenol inhibited compound 48/80-induced systemic anaphylaxis 100% with a dose of 10 micrograms g-1 body weight (BW). While serum levels of histamine were markedly elevated after compound 48/80 injection in all groups of rats, rats injected with eugenol showed a significant reduction in serum histamine levels. Eugenol also inhibited passive cutaneous anaphylaxis activated by anti-dinitrophenyl (DNP) IgE. Eugenol dose-dependently inhibited histamine release from the rat peritoneal mast cells (RPMC) activated by compound 48/80 or anti-DNP IgE. The morphological examination clearly showed that eugenol prevented the anaphylactic degranulation of RPMC. Moreover, Eugenol (10 micrograms ml-1) had a significant inhibitory effect on anti-DNP IgE-induced tumor necrosis factor-alpha production. These results suggest that eugenol has antianaphylactic properties by preventing mast cell degranulation.