There is considerable interest in the wear of polyethylene and the resulting wear-debris-induced osteolysis in artificial hip joints. Proteins play an important role as boundary lubricants in vivo in the pseudosynovial fluid, and these are reproduced in in vitro tests through the use of bovine serum. Little is known, however, about the effect of phospholipid concentrations within proteinaceous solutions on the wear of ultra-high molecular weight polyethylene (UHMWPE). The effects of protein-containing lubricants with 0.05, 0.5 and 5 per cent (w/v) phosphatidyl choline concentrations on the wear of ultra-high molecular weight polyethylene (UHMWPE) were compared with 25 per cent (v/v) bovine serum which had 0.01 per cent (w/v) lipid; the effects were compared in a hip joint simulator with smooth (n = 4) and scratched (n = 3) femoral heads. The control bovine serum lubricant produced UHWMPE wear of 55 and 115 mm3/10(6) cycles on the smooth and rough heads respectively. The increased phospholipid concentration significantly reduced the wear rate. At the higher concentration (5% w/v phosphatidyl choline) the average wear was reduced to less than 2 mm3/10(6) cycles. Even with the relatively low concentrations of 0.05% w/v phosphatidyl choline the wear was reduced by at least threefold compared with the bovine serum tests for both the smooth and rough femoral heads. There may be considerable differences in the phospholipid concentrations in patients' synovial fluid and this is highly likely to produce considerable variation in wear rates. In vitro, differences in the phospholipid concentration of lubricants may also cause variation in wear rates between different simulator tests.