The subcellular localization of the alpha 2-macroglobulin receptor, also known as the low density lipoprotein receptor-related protein (LRP), was studied in postmortem human brain tissue by light and electron microscopic immunocytochemistry. A specific monoclonal antibody (A2MR2) against the extracellular alpha-chain of the molecule was utilized. Light microscopically, LRP was detected strongly in neurons, weakly in some glial cells and discontinuously along capillary membranes. At the electron microscopic level, positive reaction products were found to be associated with plasma membranes, ribosomes, lysosomes and lipofuscin granules of neurons, glial cells and pericytes. The results suggest that LRP may have a function, particularly in neurons, of receptor-mediated endocytosis with subsequent lysosomal uptake and degradation of ligands such as alpha 2-macroglobulin proteinase complexes and apolipoprotein E.