Oxidative lipid membrane damage is known to promote the misfolding of Abeta42 into pathological beta structure. In fully developed senile plaques of Alzheimer's disease, however, it is the shorter and more soluble amyloid beta protein, Abeta40, that predominates. To investigate the role of oxidative membrane damage in the misfolding of Abeta40, we have examined its interaction with supported lipid monolayer membranes using internal reflection infrared spectroscopy. Oxidatively damaged lipids modestly increased Abeta40 accumulation, with adsorption kinetics and a conformation that are distinct from that of Abeta42. In stark contrast, pretreatment of oxidatively damaged monolayer membranes with Abeta42 vigorously promoted Abeta40 accumulation and misfolding. Pretreatment of saturated or undamaged membranes with Abeta42 had no such effect. Parallel studies of lipid bilayer vesicles using a dye binding assay to detect fibril formation and electron microscopy to examine morphology demonstrated that Abeta42 pretreatment of oxidatively damaged membranes promoted the formation of mature Abeta40 amyloid fibrils. We conclude that oxidative membrane damage and Abeta42 act synergistically at an early stage to promote fibril formation by Abeta40. This synergy could be detected within minutes using internal reflection spectroscopy, whereas a dye-binding assay required several days and much higher protein concentrations to demonstrate this synergy.