Instudies of the mechanism of action, a widespread approach is to analyse theeffect of peptides on the integrity or, more in general, the mechanicalproperties of membranes. Proteins and lipids exhibit different rheologicalproperties, with the former exhibiting high shear viscosity and/or elasticity(). We have found that the amyloidogenic peptide Aβ1-40 (40aa) exhibit high (>20 mN/m) in Langmuir monolayers. Comparatively, the26 aa Melittin (Mel) did not exhibit shear [1]. On the other hand, while bothpeptides form homogeneous monolayers, their mixtures with lipids appearheterogeneous, with peptide enriched domains (Mel) and fibrillar structures (Aβ1-40).This led us to evaluate if the rheology of mixtures is governed by thecontinuous phase. Using macro-rheological determinations, no shear was observedeven in mixtures containing high contents of Aβ1-40. The variation ofcompressibility of monolayers was used to identify phase transitions upon compression(Comp) / expansion (Exp) cycles. Mixtures containing Aβ1-40exhibit phase diagrams which differ between Comp and Exp. This hysteresis canbe related to the hysteresis observed in the pure Aβ1-40monolayer, whose wComp was 2-fold wExp. Furthermore, thishysteresis wasn´t reproducible in a second Comp/Exp cycle suggesting theestablishment of irreversible interactions. Contrary to Mel and other peptidesthat exhibit a ?lipid-like? behavior, Aβ1-40 compression isothermspresent a T dependence that suggests a non zero entropy of compression. Ourresults are discussed in terms of the interactions that can be establishedbetween peptides at molecular at which they are faced to in natural membranes.