A laboratory study has been made of the lubrication and friction characteristics of a plastic on metal (Charnley-Muller) and a metal on metal (McKee-Farrar) total hip joint replacement prostheses. The results show the effect of lubricant, speed, and load on the performance of the joints. Under a 560 lb load the coefficient of friction of the plastic to metal joint was lower than that of the metal to metal at speeds up to the equivalent of fast walking. Bovine serum and synovial fluid, as well as human serum albumin were found to be good lubricants of both types of prostheses. The frictional force produced by the metal on metal prosthesis increased linearly with load, in both the dry and lubricated states. This was not the case with the plastic to metal joint. The results show that at low physiological loads the effort required to articulate the prostheses are comparable while at higher loads the friction force of the metal to plastic is significantly lower. Friction readings taken with no lubricant indicate the serious damage which would result in the human body if a metal to metal prosthesis became devoid of liquid. The load and speed behavior of the prostheses require different interpretation of the data than presently postulated.