A soluble derivative of low-density lipoprotein receptor related protein-1 (sLRP-1) binds directly to Alzheimer's amyloid-.beta. peptide (A.beta.). This binding may be used to detect A.beta. or to separate A.beta. from the rest of a subject's body. In Alzheimer's disease, it may be used to provide diagnostic results by detecting A.beta., treatment by removing A.beta., or both.