Lubrication of mammalian joints is mediated by lubricin, a product of megakaryocyte stimulating factor gene (MSF; GenBank accession #U70136) expression. Lubricin (M_r ∼ 240 kDa) is a mucinous glycoprotein which is 50% (w/w) post-translationally modified with β(1-3)Gal-GalNAc incompletely capped with NeuAc, and lubricates apposed cartilaginous surfaces in the boundary mode through an unknown mechanism. Both bovine and human lubricin were purified from synovial fluid and digested with recombinant glycosidases. Released oligosaccharides were identified and quantified by fluorophore assisted carbohydrate electrophoresis (FACE). Corresponding digests of human lubricin were also assayed in a friction apparatus oscillating latex rubber against polished glass at a pressure of 0.35 × 10⁶ N/m² and the coefficient of friction (μ) was measured. Digestion with α2,3-neuraminidase decreased lubricating ability by 19.3%. Partial removal of β(1-3)Gal-GalNAc moieties by endo-α-N-acetyl-D-galactosaminidase reduced lubricating ability by 77.2%. Human lubricin digested with combined α2,3-neuraminidase and β1-3,6-galactosidase continued to lubricate at 52.2% of its nominal value. Both bovine and human lubricin released 48.6% and 54.4% of total β(1-3)Gal-GalNAc sidechains following digestion with endo-α-N-acetyl-D-galactosaminidase. Biological boundary lubrication by synovial fluid in vitro is provided primarily by extensive O-linked β(1-3)Gal-GalNAc.