1. Polyacrylamide beads containing entrapped 35S-labelled proteoglycan molecules have been prepared. 2. The measurement of release of radioactivity provides an extremely sensitive assay for proteoglycan-degrading enzymes, including proteinases and hyaluronidase. 3. The amount of label released is a logarithmic function of enzyme concentration or time of incubation. Experiments were made in an attempt to explain this. 4. Assays were made by the new method at several pH values, and with the inclusion of inhibitors to identify the proteoglycan-degrading enzymes of rabbit ear cartilage. 5. A previously undescribed proteinase active against proteoglycan at pH4.5 but unaffected by pepstatin, was discovered. The enzyme was named cathepsin F, and was partially purified and characterized; it was detected in human articular cartilage.