Soluble amyloid-beta peptide (Abeta) exists in the form of monomers and oligomers, and as complexes with Abeta-binding molecules, such as low-density lipoprotein receptor-related protein-1 (LRP-1) ligands. The present study investigated the effect of self-aggregation and LRP-1 ligands on the elimination of human Abeta(1-40) [hAbeta(1-40)] from the rat brain across the blood-brain barrier. Incubation of [(125)I]hAbeta(1-40) monomer resulted in time-dependent and temperature-dependent dimer formation, and the apparent elimination rate of [(125)I]hAbeta(1-40) dimer was significantly decreased by 92.7% compared with that of [(125)I]hAbeta(1-40) monomer. Pre-incubation with LRP-1 ligands, such as activated alpha2-macroglobulin (alpha2M), apolipoprotein E2 (apoE2), apoE3, apoE4, and lactoferrin, reduced the elimination of [(125)I]hAbeta(1-40). By contrast, pre-administration of the same concentration of these molecules in the rat brain did not significantly inhibit [(125)I]hAbeta(1-40) monomer elimination. Purified [(125)I]hAbeta(1-40)/activated alpha2M complex and [(125)I]activated alpha2M were not significantly eliminated from the rat brain up to 60 min. MEF-1 cells, which have LRP-1-mediated endocytosis, exhibited uptake of [(125)I]activated alpha2M, and enhancement of [(125)I]hAbeta(1-40) uptake upon pre-incubation with apoE, suggesting that [(125)I]activated alpha2M and [(125)I]hAbeta(1-40)/apoE complex function as LRP-1 ligands. These findings indicate that dimerization and LRP-1-ligand complex formation prevent the elimination of hAbeta(1-40) from the brain across the blood-brain barrier.