CEREBRAL deposition of the β-amyloid peptide (Aβ) is an invariant feature of Alzheimer's disease. Since the original isola-tion and characterization of αβ (ref. 1) and the subsequent cloning of its precursor protein2-5, no direct evidence for the actual production of discrete Aβ has been reported6-11. Here we investigate whether Aβ is present in human biological fluids using antibodies specific for an epitope within Aβ that spans the site of normal constitutive cleavage 12,13. These antibodies were used to construct a sandwich type enzyme-linked immunosorbent assay that detects Aβ in cerebrospinal fluid, plasma and conditioned medium of human mixed-brain cells grown in vitro (see also ref. 14). By affinity chromatography, we have purified and sequenced Aβ and a novel Aβ fragment from human cerebrospinal fluid and conditioned medium of human mixed-brain cell cultures. These findings demonstrate that Aβ is produced and released both in vivo and in vitro. These observations offer new opportunities for developing diagnostic tests for Alzheimer's disease and therapeutic strategies aimed at reducing the cerebral deposition of Aβ.