In orthopaedic tribology, the ISO 14242-1 standard recommends that the serum lubricant is diluted with deionized water (DW). Sometimes, phosphate buffered saline (PBS) is used instead for the dilution. Noncyclic, multidirectional pin-on-disk tests showed that the dilution of alpha calf serum with PBS or phosphate buffer resulted in a 5–40-fold decrease in the wear factor of unirradiated and of vitamin E stabilized, highly crosslinked ultrahigh molecular weight polyethylene (UHMWPE) compared with DW dilution. It appears that the phosphates were mainly responsible for this phenomenon. PBS dilution may result in a substantial underestimation of clinical wear. With 1:1 DW dilution, the mean wear factor of highly crosslinked UHMWPE was 5.7-fold lower than that of unirradiated UHMWPE.