Friction measurements were performed on fractions prepared from bovine synovial fluid by using a cartilage on glass apparatus. A fraction containing lubricating glycoprotein-I (LGP-I) as the only detectable component at concentrations of 30-50 p g / d was able to lubricate in an identical manner to whole synovial fluid. These data indicate that LGP-I is the molecule responsible for the lubricating ability of synovial fluid. '2510dine-labeled LGP-I also lubricated in a manner similar to synovial fluid, whereas when this sample was reduced and alkylated or treated with neuraminidase, the lubricating activity was greatly decreased. In tests to measure binding of ' "I LGP-I to cartilage, an initial linear increase in binding was observed, followed by a decrease in binding at higher concentrations. In contrast, both the reduced and alkylated and the neuraminidase treated samples did not show the same concentrationdependent binding to the cartilage. It is suggested, therefore, that at least part of the lubricating ability of LGP-I is dependent upon its ability to bind to articular cartilage.