7,8-Dihydroxyflavanone, isolated from the seeds of Alpinia Katsumadai Hayata, showed an inhibitory effect on Jun-Fos dimer action. 7,8-Dihydroxyflavanone blocked the action of the dimer on a DNA consensus sequence, the AP-1 binding site. We have concluded that the Jun-Fos heterodimer, bound with 7,8-dihydroxyflavanone, cannot bind to the AP-1 site and therefore results in signal interruption. The 7,8-dihydroxyflavanone was also found to have an in vitro cytotoxic effect against A549 (a human lung cancer cell line) and K562 (a human leukemia cell line).