The essential oil of Chenopodium ambrosioides L., Amaranthaceae, was obtained by steam distillation in a Clevenger apparatus and characterization was performed using chromatographic and spectroscopic assays (GC-FID, GC/MS, 1H NMR). Two major compounds were identified: p-cymene (42.32%) and ascaridole (49.77%). The ethanolic extract and hydrolate were fractionated by liquid liquid partitioning and the compounds were characterized by GC/MS. The essential oil, ethanol extract and fractions by partitioning with dicloromethane, ethyl acetate and butanol were tested in tumor cell lines (K562, NALM6, B15, and RAJI). Significant cytotoxic activity was found for essential oil (IC50 = 1.0 µg/ml) for RAJI cells and fraction dicloromethane (IC50 = 34.0 µg/ml) and ethanol extract (IC50 = 47.0 µg/ml) for K562 cells. The activity of the essential oil of C. ambrosioides is probably related to the large amount of ascaridol, since the other major compound, p-cymene, is recognized as a potent anti-inflammatory and has low cytotoxic activity. 2615661