A new triene, 12,13-dehydrogeranylgeraniol (1), was isolated from the aquatic plant Saururus cernuus and its structure determined spectroscopically. Compound 1 inhibits PMA-induced peroxide-catalyzed oxidation of 2',7'-dichlorodihydrofluorescein dye (DCFH) by reactive oxygen species (ROS) within human promyelocyctic HL-60 cells.