In an effort to carry out a more in-depth investigation on the antimicrobial properties of H. perforatum, we have assayed different extracts (MeOH; petroleum ether; CHCl(3) and EtOAc) from the aerial parts of the plant against selected microorganisms. Growth inhibition was observed only for Gram-positive bacteria, B. subtilis and B. cereus being the most susceptible to the tested drugs. The Hypericum extract obtained with EtOAc was the most active. The main constituents of this extract, as determined by HPLC analysis, were flavonoids, hypericins and hyperforins. Incubation of the selected microorganisms with the pure chemicals resulted in a significant inhibition of their growth by hypericin, hyperforin and its stable dicyclohexilammonium salt. Flavonoids appeared inactive at all.