The anti-allergic activity of a tomato extract was studied by using an in vitro histamine-release assay. The tomato skin extract exerted the strongest inhibition of histamine release. Chlorogenic acid, rutin and naringenin were identified in the 60% ethanol extract of tomato skin. However, the extract contained an unknown compound which strongly inhibited histamine release. This active compound in tomato skin was identified as naringenin chalcone (trans-2'4'6'4-tetrahydroxychalcone). Naringenin chalcone inhibited histamine release with an IC(50) value of 68 microg/ml. The anti-allergic activity of the tomato skin extract was next investigated by the in vivo mouse ear-swelling response. We found that naringenin chalcone showed the strongest inhibitory effect of the polyphenols of the tomato skin extract. These results indicate that a tomato skin extract could inhibit allergic reactions.