A beta(1-40) coated 20 nm gold colloidal nanoparticles exhibit a reversible color change as pH is externally altered between pH 4 and 10. This reversible process may contain important information on the initial reversible step reported for the fibrillogenesis of A beta (a hallmark of Alzheimer's disease). We examined this reversible color change by microscopic investigations. AFM images on graphite surfaces revealed the morphology of A beta aggregates with gold colloids. TEM images clearly demonstrate the correspondence between spectroscopic features and conformational changes of the gold colloid.