This study investigated the biological activity of etanolic extract of leaves and stem of Mimosa debilis (leguminosae). The eluate of etanolic extract (hexan, chloroform, etil acetate, acetone and methanol) were obtained for chromatography in silica gel column. The test of substance class revealed the presence of terpenes and steroids. The antibacterial activity were investigated against the gram positive microorganisms S. pneumoniae and S. aureus and the gram negative microganisms E. coli and P. aeruginosa in diffusion in disc test and dilution broth test. The diffusion in disc test was made with etanolic extract and eluate. Its was vewrified the formation of an inhibition zone only when the discs were impregned with 2,7 mg the extract per disc. All the extracts, except the metanolic eluate revealed positive results against the gram positive microorganisms. There was no formation of inhibition zone, in the used concentration, in the gram negative microorganisms culture. Dilution broth tests were made only with extracts that revealed the bests results in diffusion disc test. The eluate of etila acetate was choosen to verify the inhibition of growth in S. pneumoniae and S. aureus. The quantification was obtained for optical density. The results showed that the evaluate of leaves and stems on M debilis inhibited sigificatively the growth of S. pneumoniae and P. aeruginosa.