Biochemical markers for Alzheimer's disease (AD) are of great value for precise diagnosis and in studies of the pathogenetic processes of this disease. A new biochemical assay allowing to differentiate AD from other forms of dementia is described. The assay is based on the extraction of amyloid beta (A beta) from milligram amounts of brain tissue by using 20% acetonitrile in 0.1% trifluoroacetic acid and its detection by Western blotting. The presence of the 4 kDa A beta was demonstrated in all cases of AD (n = 8) that were diagnosed by the independent histopathological examination of the postmortem tissues. No A beta was found in tissue extracts from seven out of eight cases of other forms of dementia. In contrast to other biochemical techniques of A beta detection in brain, the developed assay is simple; it does not require any special equipment and allows detection of A beta using milligram amounts of brain tissue.