Endogenous ligands have not, to date, been identified for the asialoglycoprotein receptor (ASGP-R), which is abundantly expressed by parenchymal cells in the liver of mammals. On the basis of the rapid clearance of BSA bearing multiple chemically coupled sialic acid (Sia)α2,6GalNAcβ1,4GlcNAcβ1,2Man tetrasaccharides (SiaGGnM-BSA) from the circulation, and the ability of the ASGP-R hepatic lectin-1 subunit to bind SiaGGnM-BSA, we previously proposed that glycoproteins modified with structures terminating with Siaα2,6GalNAc may represent previously unrecognized examples of endogenous ligands for this receptor. Here, we have taken a genetic approach using wild-type and ASGP-R-deficient mice to determine that the ASGP-R in vivo does indeed account for the rapid clearance of glycoconjugates terminating with Siaα2,6GalNAc. We have also determined that the ASGP-R is able to bind core-substituted oligosaccharides with the terminal sequence Siaα2,6Galβ1,4GlcNAc but not those with the terminal Siaα2,3Galβ1,4GlcNAc. We propose that glycoproteins bearing terminals Siaα2,6GalNAc and Siaα2,6Gal are endogenous ligands for the ASGP-R, and that the ASGP-R helps to regulate the relative concentration of serum glycoproteins bearing α2,6-linked Sia.