The mastoparans are amidated tetradecapeptides, with Lys residues classically located in the positions 4, 11 and 12, which are responsible for histamine releasing from mast cells, serotonine from platelets, catecholamines and adenylic acid from adrenal chromafin cells and even causes exocytose in rat pituitary cells. The aim of the present investigation is to study the structure/activity relationship of the mastoparans presenting Lys residues in different positions of the primary sequence and the influence of charges distribution in their polyfunctional activities. The peptides were manually synthesized and purified under HPLC. The circular dichroism (CD) spectra of peptides in the presence of trifluoroethanol or sodium dodecyl sulfate revealed different percentages of a-helix: Polybia-MPI 41%; Parapolybia-MP 41%; Protonectarina-MP 39%; Apoica-MP 38%; and MK-578 23%. However, no organized structures were observed in aqueous solution. In presence of anionic liposomes Protonectarina-MP and Polybia-MPI showed the highest content of the a-helix conformation (39% for both peptides). Probably the positions of Lys residues near both extremes of the chain assist the helix stabilization. In addition, both peptides have an Asp residue, which neutralizes one of the positive charges given by Lys residues in the same side of the helix. The peptides Parapolybia-MP and Apoica-MP (22% and 7% of a-helix in presence of anionic vesicles, respectively), contain no Asp residue in their primary sequences, moreover they contain two Lys residues interacting to each other in their amphiphatic structure, destabilizing the helix. The antibiotic properties, hemolytic and degranulating activities were investigated. Although the peptide Apoica-MP hasn't the highest value of alpha helix percentage, its hydrophobic and hydrophilic faces are precisely defined in the helical wheel representation, which may explain the highest activity of this peptide among the assayed mastoparans. Thus, these results reveal that not only the content of helical conformation, but a combination of this factor with other parameters such as amphipathicity and the location of positive charges probability play an important role in peptide-membrane interaction. Supported by FAPESP