The major component of Alzheimer’s disease amyloid is a small polypeptide referred as the amyloid β protein, which is derived from a larger precursor, amyloid precursor protein (APP). Cell fractionation and immunological studies on the APP molecule indicate that APP is localized either in the neuron and astrocyte and that the molecule is recovered from lysosomal fraction. © 1993 S. Karger AG, Basel.