Phospholipase A2 (PLA2) is activated synergistically by a variety of peptides. In this study, we have investigated the nature of this synergistic mechanism using melittin (MEL) and two different synthetic amphipathic peptides bearing various numbers of positive charges. These simplified peptides containing only leucine and lysine residues were especially designed to mimic the lytic function of MEL. The measurement of the variation with time of the molecular area and surface potential on hydrolysis of phospholipid monolayers by PLA2, in the absence or presence of these peptides, shows that the rate of hydrolysis increases with the number of positive charges borne by the peptides. Moreover, epifluorescence microscopy of the labelled enzyme and peptides is used to shaw the effect of MEL or its analogues on the hydrolysis of dipalmitoylphosphatidylcholine (DPPC) monolayers in the phase transition. It is concluded that both MEL and the peptide analogues bind preferentially to the fluid phase lipids and create defects on the edges of the solid domains.