It is well known that the load of an ordinary friction slider is supported by a large number of surface asperities having a collective area that is small compared with the apparent area of contact. The metal in bulk beneath such surface asperities is elastically loaded. In many metalworking operations, such as wire drawing, extruding, rolling, and metal cutting, the bulk metal undergoes plastic deformation as sliding occurs. The influence of this subsurface flow upon the coefficient of sliding friction is discussed. A simple test procedure for studying the friction characteristics of sliding metal surfaces, one of which is being subjected to plastic flow in bulk, is described, and representative data are presented for both dry and lubricated sliding.