OBJECTIVE: To determine whether the synovial fluid (SF) constituents hyaluronan (HA), proteoglycan 4 (PRG4), and surface-active phospholipids (SAPL) contribute to boundary lubrication, either independently or additively, at an articular cartilage-cartilage interface. METHODS: Cartilage boundary lubrication tests were performed with fresh bovine osteochondral samples. Tests were performed using graded concentrations of SF, HA, and PRG4 alone, a physiologic concentration of SAPL, and various combinations of HA, PRG4, and SAPL at physiologic concentrations. Static (mu(static, Neq)) and kinetic (<mu(kinetic, Neq)>) friction coefficients were calculated. RESULTS: Normal SF functioned as an effective boundary lubricant both at a concentration of 100% (<mu(kinetic, Neq)> = 0.025) and at a 3-fold dilution (<mu(kinetic, Neq)> = 0.029). Both HA and PRG4 contributed independently to a low mu in a dose-dependent manner. Values of <mu(kinetic, Neq)> decreased from approximately 0.24 in phosphate buffered saline to 0.12 in 3,300 mug/ml HA and 0.11 in 450 mug/ml PRG4. HA and PRG4 in combination lowered mu further at the high concentrations, attaining a <mu(kinetic, Neq)> value of 0.066. SAPL at 200 mug/ml did not significantly lower mu, either independently or in combination with HA and PRG4. CONCLUSION: The results described here indicate that SF constituents contribute, individually and in combination, both at physiologic and pathophysiologic concentrations, to the boundary lubrication of apposing articular cartilage surfaces. These results provide insight into the nature of the boundary lubrication of articular cartilage by SF and its constituents. They therefore provide insight regarding both the homeostatic maintenance of healthy joints and pathogenic processes in arthritic disease.