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IMMUNOHISTOCHEMISTRY was used to assess the distribution of protein tyrosine kinases in Alzheimer's disease (AD) pathology. One of these, fyn, exhibited an interesting pattern of immunoreactivity. Low levels of immunereactivity were apparent in neurons from both normal and AD brain. However a subset of neurons in AD brain exhibited intense fyn immunoreactivity. Double label immunohistochemistry revealed that fyn-positive neurons were also positive for abnormally phosphorylated microtubule-associated protein, tau. Proline-directed kinases, whose activity is regulated by phosphorylation on tyrosine, may be responsible for abnormal tau phosphorylation. These results identify fyn as a candidate kinase for regulating putative proline-directed kinases involved in abnormal tau phosphorylation.