A standard (four-ball) test has been used to study the anti-wear capabilities of ovine synovial fluid, the phospholipid extracted from it, a synthetic synovial fluid, and the phospholipid removed from the articular surface by a lipid solvent. Despite using speeds and loads that were "extreme" by engineering criteria, i.e., several orders of magnitude in excess of physiological logical loadings, the indices of wear (mean scar diameters) were comparable to those of the best industrial lubricants (0.35 mm for the best oils to 0.8-0.9 mm for the best aqueous lubricants). Synovial fluid had a value as good (low) as 0.65 +/- 0.04 mm, whereas an artificial synovial fluid synthesized from hyaluronic acid and the proposed active ingredient (disaturated phosphatidylcholine) gave a mean value of 0.59 mm. These results are discussed as consistent with the hypothesis that the joints are lubricated by oligolamellar phospholipid as a lamellated solid (graphite-like) lubricant adsorbed onto the articular surface or otherwise deposited from synovial fluid. This lubricant imparts not only phenomenal anti-wear properties, but also the remarkable reduction in friction reported previously.