Quantitative friction force measurements were made using a Surface Forces apparatus (SFA) modified for friction studies of a smooth poly(n-butyl methacrylate) (PnBMA) surface sliding on a stationary mica surface, and the results are compared with those of the complementary system (ref 1) of mica sliding on a stationary polymer surface. The results indicate that qualitatively and quantitatively different tribological behavior may be expected-as manifested by smooth or stick-slip sliding, high or low steady-state friction stresses, transient effects such as stiction, and surface deformations and wear-for such "asymmetric" surfaces depending on whether the sliding or stationary surface carries the lubricant or polymer layer, on whether the motion is continuous in one direction or cyclic (back and forth), and on the area of the contacting junctions. None of these effects would arise if the friction forces were determined solely by the instantaneous load (pressure) and velocity (shear rate) of the shearing junctions, which is generally assumed in conventional theories or models of friction.