A Trichoderma koningii strain collected in Uruguay produced two antibiotic membrane-active peptidic groups, the trikoningins KA and KB, from which the main components KA V, KB I and KB II were purified. The peptide sequences were elucidated by FAB mass spectrometry and high-field NMR experiments. KA V was a nonadecapeptaibol, whereas KB I and KB II were two 11-residue lipopeptaibols. They all displayed antibiotic activity against Staphylococcus aureus. Their membrane-modifying properties were examined by following the leakage of liposome-entrapped carboxyfluorescein. KA V induced similar permeability modifications to those exhibited by known 19-residue peptaibols, while the KB I and KB II activities were weaker.