Joint cartilage consists of cells embedded in a matrix of fibrous collagen within a concentrated water-proteoglycan gel. The integrity of this matrix is crucial for the biomechanical properties of the joint cartilage. The different components of the matrix are synthesized and degraded by the cartilage cells, a process regulated by the amount of mechanical stress applied to the chondrocytes as well as by peptide factors and hormones present in synovial fluid. The proteoglycans are large macromolecules consisting of a protein core to which are attached multiple chains of glycosaminoglycans and oligosaccharides. During normal and pathological turnover, degradation products are released to the synovial fluid and to the circulation. Newly developed assays allow the sensitive and specific detection of these fragments in joint fluid and serum. Results of experimental and clinical investigations suggest that these assays will be of value in efforts to diagnose, grade and predict the outcome of inflammatory and degenerative joint disease.