The rate of racemization of aspartic acid was measured in young and aged human femoral head cartilage. Normal femoral heads were obtained at postmortem, osteoarthritic specimens at operations for total hip replacement. In order to distinguish between the aspartic acid racemization in collagen from that in proteoglycan (PG), in addition to native tissue, we tested cartilage specimens from which PG had been enzymatically removed. Preliminary results indicate that there is only a very slow collagen turnover in normal adult cartilage. The same is true of residual cartilage from osteoarthritic femoral heads, indicating no rapid repair except where osteophytes are formed. Native, PG-containing cartilage, whether normal or osteoarthritic was found to have unexpectedly high racemization rates.