Amyloid beta-protein, or beta/A4, is a 4-kilodalton peptide that forms poorly soluble extracellular depositions of amyloid in brains and leptomeninges of patients with Alzheimer's disease (AD), Down's syndrome (DS), and hereditary cerebral hemorrhage with amyloidosis-Dutch type (HCHWA-D). beta/A4 peptide is a derivative of a large transmembrane glycoprotein (APP) and is found in the extracellular space, i.e., in the cerebrospinal fluid and serum of individuals with and without AD and in the conditioned media of many different cells grown in culture. The mechanism by which normally produced amyloid beta peptide forms extracellular aggregates in patients is unknown. One possible explanation is a failure of a mechanism for removal of the beta/A4 peptide that prevents this highly aggregating peptide from forming extracellular amyloid depositions.