Proteoglycan fragment concentrations in joint fluid of rabbit knees with various degrees of secondary arthrosis after meniscal resection, meniscal substitution, and anterior cruciate ligament transection, were related to the gross appearance of the articular cartilage. Knees with normal cartilage were distinguishable by the concentration of proteoglycan fragments in joint fluid from knees with arthrosis. We found a correlation (r 0.61) between increasing arthrosis and increasing concentrations of proteoglycan fragments.