beta-Amyloid (Abeta), a 39-43 residue peptide, is the principal component of senile plaques found in the brains of patients with Alzheimer's disease (AD). There are two main lines of evidence that its deposition is the cause of neurodegeneration. First, mutations found in three genes in familial Alzheimer's cases give rise to increased production of the longest, most toxic, form, Abeta 1-42. Second. aggregated Abeta is toxic to neuronal cells in culture. Inhibitors of the proteases involved in its release from the amyloid precursor protein are, therefore, of major therapeutic interest. The best candidates for the releasing proteases are both aspartyl proteases, which are integrated into the membranes of the endoplasmic reticulum and Golgi network. A sensitive assay using Ciphergen's Seldi system has been developed to measure all the variants of Abeta in culture supernatants, which will be of great value in screening inhibitors of these proteases. With this assay, it has been shown that increasing intracellular cholesterol increases the activities of both beta-secretase, and gamma-secretase 42. Moreover, changing the intracellular targeting of amyloid precursor glycoprotein (APP) yields increased alpha-secretase cleavage, and increases in the amounts of oxidized/nitrated forms of Abeta.