Berberine sulfate, an isoquinoline alkaloid isolated from Hydrastis canadensis L., inhibited the effects of the tumor promoters 12–O-tetradecanoylphorbol-13-acetate and teleocidin, such as increased 32Pi-incorporation into phospholipids of cell membrane and hexose transport. Berberine sulfate also markedly suppressed the promoting effect of teleocidin on skin tumor formation in mice initiated with 7,12-dimethyl-benz[a]anthracene.