In the course of our continuing search for novel cancer chemopreventive agents from natural sources, several kinds of Labiatae plants were screened. Consequently, the iridoid glycoside derivative, 8-acetylharpagide (8-AcHarp), was obtained from the flowering whole plant of Ajuga decumbens as an active constituent. This glycoside exhibited the remarkable inhibitory effect on two-stage carcinogenesis test of mouse skin tumors induced by nitric oxide (NO) donor, (+/-)-(E)-methyl-2-[(E)-hydroxyimino]-5-nitro-6-methoxy-3-hexen eamide (NOR 1) as an initiator and 12-O-tetradecanoylphorbol-13-acetate (TPA) as a promoter. Further, 8-AcHarp exhibited potent anti-tumor-promoting activity on two-stage carcinogenesis test of mouse hepatic tumor using N-nitrosodiethylamine (DEN) as an initiator and phenobarbital (PB) as a promoter.