A hallmark of Alzheimer's disease is the accumulation of insoluble fibrils in the brain composed of amyloid beta (A beta) proteins with parallel in-register cross-beta-sheet structure. It has been suggested that the aggregation of monomeric A beta proteins into fibrils is promoted by "seeds" that form within compartments of the brain that have limited solvent due to macromolecular crowding. To characterize these seeds, a crowded macromolecular environment was mimicked by encapsulating A beta 40 monomers into reverse micelles. Fourier-transform infrared spectroscopy revealed that monomeric A beta proteins form extended beta-strands in reverse micelles, while an analogue with a scrambled sequence does not. This is a remarkable finding, because the formation of extended beta-strands by monomeric A beta proteins suggests a plausible mechanism whereby the formation of amyloid fibrils may be nucleated in the human brain.