Alzheimer's disease is characterized by the deposition of beta-protein (Abeta) as amyloid. Recently, it was found that Abeta is a normal component of serum and cerebrospinal fluid. Synthetic peptides homologous to Abeta form amyloid-like fibrils spontaneously in water or physiological solutions. Using a peptide homologous to Abeta1-40, we find that fibril formation is inhibited by the presence of cerebrospinal fluid.