The purpose of the present study was to determine the usefulness of the monoclonal antibodies 7‐D‐4 and 3‐B‐3 as biomarkers of severity of naturally occurring osteoarthritis in the knee joints of adult cynomolgus macaques. The antibodies were used to immunolocate chondroitin sulfate proteoglycan epitopes in articular cartilage or synovial fluid from knee joints with a range in severity of osteoarthritis. The joints were examined radiographically, grossly, microradiographically, and histologically to characterize the severity of disease, and the results of three different methods of proteoglycan analysis (immunohistochemistry, enzyme‐linked immunosorbent assay, and Western blot analysis) were compared. Subjectively, the degree of positive immunostaining for 7‐D‐4 was minimal in normal sites and increased as damage to articular cartilage increased. The scores for 7‐D‐4 immunostaining in the medial tibial plateau (the site most severely involved in this model) were correlated significantly with severity of damage to articular cartilage (p < 0.05, r2 = 0.50), thus supporting the subjective observations. The ratio of 7‐D‐4 to sulfated glycosaminoglycans in synovial fluid also was correlated with the score for 7‐D‐4 immunostaining in the medial tibial plateau (p < 0.05, r2 = 0.54) and with the score for 3‐B‐3 immunostaining in the medial femoral condyle (p < 0.05, r2 = 0.65). There were no significant correlations among scores for 3‐B‐3 immunostaining, severity scores, and the ratios of 3‐B‐3 to sulfated glycosaminoglycans in the synovial fluid. By Western blot analysis, both epitopes were sensitive markers of early cartilage damage in young adult monkeys but were less sensitive in older monkeys. This work provides evidence that measurement of the epitope recognized by 7‐D‐4 in synovial fluid or, by immunohistochemical or Western blot methods, in articular cartilage has potential use as a marker of severity of naturally occurring osteoarthritis.