Induction of phase II enzymes is a major mechanism of chemoprevention. The induction levels of quinone reductase (QR) activity in cultured murine hepatoma (Hepa 1c1c7) cells by 80%-methanol extracts of traditional medicinal plants were measured. Among the tested 81 plants, the extracts of Aralia continentalis, Magnolia obovata, and Viscum album were found to induce QR activities over 250%. The maximum induction levels obtained were 401.9%, 270.5%, and 301.8% by treatments of the extracts of A. continentalis $(318;{mu}g/ml)$, M. obovata $(53.8;{mu}g/ml)$ and V. album $(80.6;{mu}g/ml)$, respectively. These QR induction activities were more potent than those of the known QR inducers, t-butylhydroquinone (170.1%) and ${eta}-naphthoflavone$ (320.0%).