The effect of biological molecules, such as lipids and proteins, contained in physiological environment on the wear of prosthetic hip joint materials was investigated. In vitro wear test were carried out on a medical grade alumina ceramics as a bearing material for ceramic-on-ceramic hip joint prosthesis. The wear behavior of ultra high molecular polyethylene (UHMPE)-on-metal sliding pair for hip joint prosthesis was also evaluated by using a multidirectional sliding pin-on-plate wear tester. The wear rate of alumina-on-alumina sliding pairs in diluted bovine serum was found to be lower than that in distilled water. On the other hand, wear of UHMWPE was accelerated in bovine serum.