MUCH recent work^1–4 has shown that a considerable proportion of the chondroitin sulphate in the ground substance of cartilage is combined in some way or other with protein. It is possible that the relative ease with which the mucopolysaccharide can be obtained in a protein-free form⁵ by extraction with mild alkali is a reflexion of the alkali lability of some of the protein-mucopolysaccharide bonds⁴. It is also known that papain and trypsin^2,4 lower the viscosity of solutions of chondroitin sulphate–protein complex and partially remove the proteins. Plasmin is a protease resulting from the activation of serum plasminogen by diverse means, including the action of the so-called kinases of streptococci⁶ and staphylococci^7,8. In view of the chondrolytic effects of joint infections caused by staphylococci and streptococci which produce kinases, in contrast to infection by other organisms, such as the tubercle, which do not produce kinases, it seemed important to see whether plasmin preparations would liberate chondroitin sulphate from cartilage in vitro: such a liberation might be a step in destruction of cartilage in vivo.