The cell viability of Lepechinia speciosa (St. Hill) Epling fractions was measured by cell membrane integrity (lactate dehydrogenase assay) on rat basophilic leukemia cells (RBL-2H3). All fractions and extract tested (100 μg/ml) increased the release of lactate dehydrogenase (LDH), being the ethyl acetate and dichloromethane fractions with LDH release of 94.5% and 91.2%, respectively. As these fractions showed decrease of cell viability, the antiproliferative activity on human breast adenocarcinoma cells (MCF-7) through sulphorhodamine B (SRB) assay was performed with them. The dichloromethane fraction (50 μg/ml) displayed the maximum activity (95% of inhibition) (IC50 = 1.99 ± 0.06 μg/ml). From this fraction was obtained a mixture containing two triterpenes (ursolic and oleanolic acids) and one fatty acid (palmitic acid), which were identified by gas chromatography coupled to mass spectrometry (GC-MS) and had their structures confirmed by 13C NMR. Rosmarinic acid and verbascoside were isolated from the ethyl acetate fraction and had their structures confirmed by 1H NMR.Colegio de Farmacéuticos de la Provincia de Buenos Aire
