The leaves of poplar trees (P. nigra and P. deltoides) contain antimicrobial agents. These antibiotic properties are markedly influenced by interspecific and intraspecific hybridizations. Fractionation of the weakly active crude extracts results in more active antibacterial fractions. There are indications that the substances which inhibit antimicrobial effects during the testing procedure remain in the methanol fractions. The antibacterial agents differ from the antifungal compounds, which both are different from the antiviral substances.