recently, several plants have been evaluated not only for antimicrobial activity but also for resistance-modifying action. in this work, the chemical composition and antibacterial and antibiotic-modulatory activities of the essential oils from lantana camara l. and lantana montevidensis briq. were analyzed. the essential oils extracted from the leaves by hydrodistillation and analyzed by gas chromatography (gc) and gas chromatography-mass spectrometry (gc-ms) were characterized by a high percentage of sesquiterpene hydrocarbons. among the 34 constituents identified, bicyclogermacrene (19.4%), isocaryophyllene (16.7%), valencene (12.9%) and germacrene d (12.3%) were the main constituents of the oil from l. camara, while in the oil from l. montevidensis, -caryophyllene (31.5%), germacrene d (27.5%) and bicyclogermacrene (13.9%) predominated. the essential oils were examined for antibiotic activities alone and in combination with aminoglycosides by a microdilution assay utilizing five bacterial strains. they exhibited significant antibacterial activities, mainly against proteus vulgaris (mic 64 g/ml, l. camara; mic 128 g/ml, l. montevidensis). both oils also showed a synergistic effect on the activity of aminoglycoside antibiotics. thus, the essential oils of l. camara and l. montevidensis could be used as a source of plant-derived natural products with resistance-modifying antibacterial activity.