The secreted form of β-amyloid precursor protein (APP) containing the Kunitz proteinase inhibitor (KPI) domain, also called protease nexin II, is internalized and degraded by cells. We show that the low density lipoprotein (LDL) receptor-related protein (LRP) is responsible for the endocytosis of secreted APP. APP_s770 degradation is inhibited by an LRP antagonist called the receptor-associated protein (RAP) and by LRP antibodies and is greatly diminished in fibroblasts genetically deficient in LRP. APP_s695, which lacks the KPI domain, is a poor LRP ligand. Since LRP also binds apolipoprotein E (apoE)-enriched lipoproteins and inheritance of the 0 allele of the apoE gene is a risk factor for Alzheimer's disease (AD), these data link in a single metabolic pathway two molecules strongly implicated in the pathophysiology of AD.