Boundary friction between two crossed nylon 6,6 fibers has been reduced up to 6x by grafting polymeric lubricants to the surface. A modified adhesion model of friction, accommodating multiple materials on the surface, explains the variation in the friction coefficient with the molecular weight of the lubricant. The friction coefficient is proportional to the fraction of the surface covered by the grafted chain, represented by pi R-G(2)/A, where A is the surface area per graft site, and R-G is the radius of gyration of the lubricant. It is also shown that for larger the grafted chains, the appropriate surface area fraction depends on pi R-G(2)/3A. It is argued that if R-G of the grafted lubricant is larger than half the distance between graft sites, the next nearest graft site is used due to steric blocking of the nearest graft site.