As an index of lysosomal enzyme activity in cartilage from twenty osteoarthritic and twelve normal joints, the acid phosphatase activity was determined and correlated with the histochemical-histological grading of the severity of the arthritis. The activity was increased four to five times normal in the arthritic cartilage with a positive correlation between activity and severity of the disease and was also increased in the cartilage of the osteophytes, possibly due to immaturity. This finding supports current concepts of the role of lysosomal enzymes in osteoarthritis.