The effect of viscosity (Z) of several lubricants, the frequency of oscillation (N) and loading (P) on the coefficient of friction (μ) of excised dog ankle joints have been studied in vitro. Bovine synovial fluid reduced μ from 0.0099, obtained when saline solution was the bath fluid, to 0.0044. Viscosity per se was not an important factor inasmuch as treatment of synovia and synovia concentrate with testicular hyaluronidase, reduced Z from between 5 and 107 centipoise to approximately that of saline, but did not affect μ. The synovial lubricating factor remained in the mucin fraction which was retained by a Millipore filter having a pore dia. of 0.22 microns, and was shown to plate out onto the surface of the cartilage. Viscous purified hyaluronic acid lacked lubricating ability. These data and the relation between μ and a variation in ZN/P indicate that combinations of self-pressurized hydrostatic, boundary and elastohydrodynamic mechanisms are involved in the lubrication of animal joints.