The interaction of the synthetic multiple antigenic peptide construct MAP(4)-VP1 (11-25) with lipid membranes was studied by a combination of spectroscopic and biophysical techniques. MAP(4)-VP1 showed an important surface activity when spread in an air-water interface, indicative of an amphipatic conformation. Since this peptide has a net anionic charge, we studied the degree of interaction with model membranes of zwitterionic dipalmitoylphosphatidylcholine (DPPC), anionic phosphatidylinositol (PI and DPPC/PI 9:1), or cationic stearylamine (SA and DPPC/SA 9:1), MAP(4)-VP1 was readily soluble in aqueous buffer, yet spontaneously inserted from an aqueous subphase into cationic and zwitterionic monolayers up to limiting pressures of 20-30 mN/m. The induced surface pressure increases were in the order cationic > zwitterionic > anionic monolayers. This indicates that the interaction is mainly driven by electrostatic forces, although there is an important hydrophobic component, responsible for the penetration in monolayers of neutral DPPC. Interaction of MAP(4)-VP1 with unilamellar vesicles of DPPC, DPPC/SA (9:1), and DPPC/PI (9:1) labeled with 1,6-diphenyl-1,3,5-hexatriene (DPH) or with 1-anilino-8-naphthalene sulfonic acid (ANS) was determined by changes in fluorescence polarization as a function of temperature, Results showed that the presence of the positively charged SA in the membrane strongly enhances the incorporation of MAP(4)-VP1 and that the peptide increases the fluidity of the bilayer and decreases the temperature of the phase transition.