The 39-43-amino acid amyloid beta-protein (A beta), which is progressively deposited in cerebral plaques and blood vessels in Alzheimer's disease (AD), is released by cultured human cells during normal metabolism. Here we show that agents which activate protein kinase C or otherwise enhance protein phosphorylation caused a substantial decrease in A beta production in vitro. Protein kinase C activation also markedly decreased A beta release from cells that express mutant forms of the beta-amyloid precursor protein genetically linked to familial AD. Inhibition of A beta secretion could also be effected by direct stimulation of m1 muscarinic acetylcholine receptors with carbachol. These results demonstrate that activation of the protein kinase C signal transduction pathways down-regulates the generation of the amyloidogenic A beta peptide. Pharmacologic agents that activate this system, including a variety of first messengers, could potentially slow the development or growth of some A beta plaques during the early stages of AD.