The term 'proteoglycan' was introduced in 1967 (see Balazs. 1970) to describe the family of molecules in which glycosaminoglycan chains are linked to protein. However. their rate of diffusion into textbooks of biochemistry has been rather slow, and I will therefore begin this presentation with a general introduction, which I hope will set the basis for their structural identity and indicate how their properties may be of importance in a broad biological context, before describing in more detail studies with proteoglycans from cartilage. Glycosaminoglycans are characteristic components of the extracellular space of vertebrate tissues. They are long unbranched polysaccharide chains which contain many acidic, carboxylate and/or sulphate groups. They do not normally occur as free chains in uiuo, but as proteoglycans, in which many 254,1312-1318 Bwl. in the press