DECORIN is a small chondroitin–dermatan sulphate proteoglycan consisting of a core protein and a single glycosaminoglycan chain^1–3. Eighty per cent of the core protein consists of 10 repeats of a leucine-rich sequence of 24 amino acids^2,4. Similar repeats have been found in two other proteoglycans, biglycan⁵ and fibromodulin⁶, and in several other proteins including Drosophila morphogenetic proteins^7–11. Expression of high levels of decorin in Chinese hamster ovary cells has a dramatic effect on their morphology and growth properties¹. We now report that this effect is due at least in part to the ability of decorin to bind transforming growth factor-β, an autocrine factor^12,13 that stimulates the growth of Chinese hamster ovary cells. As transforming growth factor-β induces synthesis of decorin in many cell types^14,15, our results suggest that decorin may be a component of a feedback system regulating cell growth.