The a beta peptide is a neurotoxic peptide that accumulates in the brains of Alzheimer patients, but is also present in body fluids at subnanomolar levels. The potential effects of these low levels of a beta are unclear. We now show that one such action is to increase tyrosine phosphorylation in PC12 cells and olfactory neuroblasts. Application of a beta 25-35 or a beta 1-40 induces a dose-dependent increase in the tyrosine phosphorylation in both whole cells and in vitro. The increase in tyrosine phosphorylation is both rapid and sensitive, being stimulated by picomolar doses of a beta and occurring within 1 min of application. Calcium imaging experiments provide further support for the role of tyrosine phosphorylation in the action of a beta. While a beta does not alter calcium metabolism under basal conditions, the addition of a beta induces a rapid increase in cytoplasmic calcium in olfactory neuroblasts that have been treated with the tyrosine phosphatase inhibitor, sodium orthovanadate or in PC12 cells treated with nerve growth factor. These responses could be blocked by the tyrosine kinase inhibitor, herbimycin. These calcium responses displayed an obligate requirement for the presence of matrix proteins. The identification of a rapid, sensitive assay for the action of a beta may facilitate investigations of its mechanism of action.