Water extract from Prunella vulgaris L. (PVW) was tested for colon cancer chemopreventive activity by measuring the activities of cytochrome P450 1A1, phase Ⅱ detoxification enzyme [quinone reductase (QR) and glutathione S-transferase (GST)] and ornithine decarboxylase (ODC) and glutathione (GSH) levels in cultured human colorectal adenocarcinoma HT-29 cells. PVW significantly inhibited 7,12-dimethylbenz[a]anthracene (DMBA)-induced cytochrome P450 1A1 activity at 10 and 50 ${mu}g/ml$. PVW induced QR activity in a dose-dependent manner over a concentration range of $1{sim}50;{mu}g/ml$. GST activity was also induced with the treatment of PVW in HT-29 cells. In addition GSH levels were increased with PVW. PVW inhibited ODC activity, a key enzyme of polyamine biosynthesis, which is enhanced in tumor promotion. These results suggest that Prunella vulgaris L. has colon cancer chemopreventive activity by inhibiting cytochrome P450 1A1 and ODC activities and by increasing phase Ⅱ enzyme activity and GSH levels.