Deposition of β-amyloid protein (Aβ) in senile plaques and in the walls of cerebral vessels is a pathologic hallmark of Alzheimer's disease (AD). The current diagnostic criteria for AD requires the presence of neurofibrillary tangles and a minimum number of senile plaques in cortex. Senile plaques are readily visualized by silver staining or immunocytochemistry using antibodies raised to Aβ. Available histochemical and immunocytochemical methods are sensitive but the results may occasionally be variable and sampling from many brain regions is difficult and impractical. This study describes a simple biochemical method for quantifying the Aβ load in unfixed brain homogenates. The immunoassay recognizes all forms of Aβ deposits (neuritic and diffuse plaques, and cerebrovascular amyloid) and has a sensitivity and specificity comparable to immunocytochemistry. In direct comparisons, results from the dot blot method correspond well with both Western blot analysis of partially purified Aβand plaque counting by immunocytochemistry. In a retrospective series of 39 postmortem AD and control cases, the amount of Aβ in brain by dot blot immunoreactivity effectively separated the two groups. Therefore, this method provides a rapid, sensitive, and accurate quantitation of Aβ in postmortem brain tissue and represents an alternative approach for studying Aβ deposition in aging and AD.