The effect of the saturation of fatty acid (FA) in 1,2-dimyristoyl-sn-glycero-3-phosphocoline (DMPC)/FA membrane on the interaction between lipid membrane and amyloid beta monomer was investigated by using the Langmuir monolayer technique. The surface pressure (Pi)-mean molecular area (A) isotherms and fluorescent measurements reveal that DMPC and octadecanoic acid (stearic acid, SA) molecules were somewhat miscible in the mixed membrane, which was maintained to homogeneous gel phase by enhance of the intermolecular hydrophobic interactions because of the all trans acyl chains. On the other hand, DMPC and 9Z,12Z-octadecadienoic acid (linoleic acid, LA) molecules were considered to be well miscible in the mixed membrane, where the membrane partially transferred from gel phase to liquid-crystalline phase. The Pi-A isotherms of the monolayers on amyloid beta-peptide (A beta) solution indicated that A beta monomers tend to be inserted into the saturated acyl chain region of monolayers at low surface pressure and that the A beta monomers were then extruded from the monolayer at higher surface pressure. It was observed that behaviors of A beta monomers at higher surface pressure depended on membrane microstructures. In the DMPC/SA monolayers, A beta aggregated and then was extruded from monolayers at about 20 mN m(-1) of surface pressure irrespective of the SA proportion. On the other hand, in the DMPC/LA monolayers, A beta, which favors to interact with DMPC, is dispersed in the monolayer even at high surface pressure because DMPC and LA molecules were well miscible in the monolayer. (C) 2017, The Society for Biotechnology, Japan. All rights reserved.