Abstract Natural products from plants can alter the effect of antibiotics, by increasing or reducing
its activity.Lippia sidoidesCham. (Verbenaceae) is a bush found in the Northeastern region of Brazil. In popular medicine,L. sidoideshas been used as antiseptic. In this work we report the chemical
composition of the essential oil fromL. sidoidesand to determine the potentiation of the activity of
aminoglycosides andb-lactams. The essential oil of L. sidoideswas extracted by hydrodistillation
using a Clevenger. The composition of the essential oil was determined by GC/MS. The determination of the Minimum Inhibitory Concentration (MIC) and the modulatory activity was realized
using microdiluition method. The essential oil showed as major compounds the thymol (84.9%),
Etil-methyl-carvacrol (5.33%) and p-cymene (3.01%). The determination of the Minimum Inhibitory Concentration (MIC) by microdiluition demonstrated no difference in the antimicrobial activity between the essential oil (EOLS) and thymol. There was reduction in the MIC of the
amynoglicosides gentamicin and neomycin when associated to EOLS and thymol. When the natural