The role of protein and phospholipid in the boundary lubrication mode of a natural synovial joint was examined by using a pendulum friction tester with a pig shoulder joint as the sliding pair. Detergent (polyoxyethylene p-t-octylphenyl ether) was successfully used to remove adsorbed film from the articular cartilage surface. The hydrophobic surface of natural articular cartilage was changed to hydrophilic by ultrasonic rinsing in the saline solution of detergent without large change of surface profile observed by confocal laser microscope. Although the coefficient of friction in the lubricating condition with the saline solution of 1.0 wt% and 0.5 wt% sodium hyaluronate was not changed by the treatment, it was significantly increased in the lubricating condition of 0.2 wt% sodium hyaluronate. This indicates that the adsorbed film on the articular cartilage surface contributes to boundary lubrication with lowerviscosity lubricant. Moreover, it was confirmed that the surface treated with detergent was significantly well lubricated with addition of 3.0 wt% γ-globulin in lubricant or 10.0 mg dipalmitoyl phosphatidylcholine on the articular cartilage surface. It appears that protein and phospholipid can be effective boundary lubricants in natural a synovial joints.