The combined use of an ultrahigh vacuum tribometer and a number of surface science techniques has enabled us to explore the tribological properties of interfaces between single-crystal metal surfaces and to address the fundamental issue of frictional anisotropy. Friction measurements have been made between a pair of Ni(100) surfaces which were prepared to be truly clean or modified by the presence of adsorbed atomic sulfur with and without adsorbed ethanol. Measurements made with systematic variation of the relative crystallographic orientations of the two Ni(100) surfaces have revealed that the friction coefficient is anisotropic with respect to lattice orientation. When aligned (theta similar to 0 degrees) and sliding along the [110] direction, the friction coefficient between the two clean Ni(100) surfaces was mu (s) = 816 +/-2.5. The minimum static friction coefficient occurred when the two clean Ni(100) surfaces were misoriented by theta similar to 45 degrees (mu (s) = 2.5 +/- 1). This orientational anisotropy is consistently observed even in the presence of adsorbed atomic sulfur and up to 4 monolayers of adsorbed ethanol, although these modified surfaces no longer have the same surface lattice periodicity as the Ni(100) substrate. The effect of lattice orientation is damped out at the point that the surfaces are separated by >20 monolayers of adsorbed ethanol. The friction anisotropy observed between Ni(100) surfaces suggests that surface lattice commensurability is not the only cause of friction anisotropy in this system.