The neuritic plaque in the brain of Alzheimer's disease patients consists of an amyloid composed primarily of Aβ, an approximately 4-kDa peptide derived from the amyloid precursor protein. Multiple lines of evidence suggest that Aβ plays a key role in the pathogenesis of the disease, and potential treatments that target Aβ production and/or Aβ accumulation in the brain as β-amyloid are being aggressively pursued. Methods to quantitate the Aβ peptide are, therefore, invaluable to most studies aimed at a better understanding of the molecular etiology of the disease and in assessing potential therapeutics. Although other techniques have been used to measure Aβ in the brains of AD patients and β-amyloid-depositing transgenic mice, the enzyme-linked immunosorbent assay (ELISA) is one of the most commonly used, reliable, and sensitive methods for quantitating the Aβ peptide. Here we describe methods for the recovery of both soluble and deposited Aβ from brain tissue and the subsequent quantitation of the peptide by sandwich ELISA.