In its various embodiments, the invention provides myelin basic proteins and fragments of that interfere with the fibril lization of peptides implicated in the amyloidoses, especially the amyloid-beta peptide associated with Alzheimer's disease (“AD”) and cerebral amyloid angiopathy (“CAA”). Some embodiments provide methods of identifying additional interfering fragments. Others provide methods of identifying substances that modulate the interference. Further embodiments provide methods of preventing amyloidoses, especially AD and CAA by administering myelin basic proteins or fragments thereof.