A number of Aß protein variants, differing only at their carboxy terminus (1-39, 1-40, 1-42 and 1-43), are identified as the major components of the cerebral amyloid deposits in Alzheimer’s disease. The length of the C-terminus is a critical determinant of the rate of amyloid formation (“kinetic solubility”), with only a minor effect on the thermodynamic solubility. Amyloid formation by the kinetically soluble peptides (e.g. 1-39) can be nucleated, or “seeded” by peptides which include the critical C-terminal residues (1-42, 26-42, 26-43, 34-42).