Eumenine mastoparan-AF (EMP-AF) is a novel membrane active tetradecapeptide recently isolated from the venom of solitary wasp, Anterhynchium flavomarginatum micado. It was reported previously that EMP-AF peptide presented low cytolytic activities in human erythrocytes and in RBL-2H3 mast cells. In the present work, we observed that this peptide is able to permeate anionic liposomes, and in less extension also the neutral ones. We present evidences showing that the permeation ability is well correlated with the amount of helical conformation assumed by the peptides in these environments. This peptide also showed a broad-spectrum inhibitory activity against Gram-positive and Gram-negative bacteria. The permeability of liposomes and the antibiotic effect showed a significant reduction when C-terminus was deamidated (in acidic form). The removal of the three first amino acid residues from the N-terminus rendered the peptide inactive both in liposomes and in bacteria. The results suggest that the mechanism of action involves a threshold in the accumulation of the peptide at level of cell membrane.Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)