One of the neuropathological lesions characteristic of Alzheimer disease (AD) is the cerebral accumulation of the amyloid β-peptide (Aβ). Although numerous studies have demonstrated that Aβ spontaneously forms amyloid in vitro, the molecular events underlying Aβ amyloid formation in vivo are less well understood. Immunohistochemical studies have shown that other proteins colocalize with Aβ in amyloid deposits in brain. The identity of one of these proteins, AMY, has so far remained elusive; therefore we attempted to purify AMY. The AMY protein was found to co-purify with Aβ in insoluble fractions from human AD brain, and was absent in brains from control subjects. AMY immunoreactivity was primarily restricted to a 50-kDa and 100-kDa protein species. Interestingly, the chromatographic and immunological profile of AMY resembled the recently identified amyloid-associated protein CLAC, derived from a transmembrane collagen-like precursor, CLAC-P. Antibodies against AMY recognized CLAC-P expressed in mammalian cells. In addition, side-by-side comparisons of AD brain sections and extracts, using antibodies against both AMY and CLAC, respectively, resulted in almost identical staining patterns. Therefore, we conclude that the AMY immunoreactivity seen in association with amyloid in AD brain is due to the presence of the CLAC protein.