Resorption of cartilage matrix is usually considered to be due to extrinsic proteases, generally thought to be the neutral metalloproteases, including collagenase. Such enzymes can be secreted from synovial tissue and in acute forms of arthritis they are believed to be the main agents of pannus erosion. However, such enzyme secretion has not been clearly demonstrated to be the causative agent in either rheumatoid arthritis or in osteoarthritis. For example, no specific inhibitors of these proteinases have yet been proved effective in vivo. Recent experiments at the Strangeways Research Laboratory have demonstrated that viable chondrocytes of animal and human articular cartilage are capable of resorbing their surrounding matrix without the aid of extrinsic enzymes. We now know that such resorption can be stimulated by the action of a low molecular weight peptide released from living synovial and other connective tissues. These messengers (catabolins) are capable of stimulating chondrocytes to degrade totally both proteoglycan and collagen. The purification, properties, regulation and action of catabolism are under investigation to determine the role for catabolin in arthritis.