The effect of immobilization of the knee in flexion on the development of osteoarthritis was studied in dogs that had undergone transection of the anterior cruciate ligament. Knees of dogs that were permitted ad libitum ambulation in a pen for 12 weeks after transection of the ligament showed osteophytes and fibrillation and a decrease in proteoglycan (uronic acid) content, although thickness of the articular cartilage was normal. Proteoglycan synthesis was 80% greater (P < 0.01) than that in cartilage from the contralateral knee, and more than twice the normal proportion of the total proteoglycans was present in the medium of cultures of cartilage from the unstable knee (P < 0.01). Similarly, the proportion of total tissue proteoglycans extracted with 0.4M guanidinium chloride was 3 times greater than normal (P < 0.01). In contrast, osteophytes were not seen when the leg was immobilized in flexion immediately after transection of the ligament, and the articular surface remained intact. The cartilage became atrophic, however, and its proteoglycan content (uronic acid) was diminished; proteoglycan synthesis was decreased to 61% of the level in cartilage from the contralateral knee, although proteoglycan extractability was normal. Knee cartilage from these legs that were immobilized after transection, therefore, resembled that from dogs whose leg had been immobilized without cruciate ligament transection. In cartilage from all operated and/or immobilized joints the proportion of proteoglycans that interacted with hyaluronic acid in vitro was diminished, because of a defect in the hyaluronate-binding region of the proteoglycan.