Abstract: Decorin, a small collagen-binding dermatan sulfate proteoglycan, is widely distributed as a component of extracellular matrices. Using a solid phase binding assay, the inventors demonstrated that decorin bound C1q at physiologic pH and ionic strength. The interaction did not require divalent cations and was time and temperature dependent reaching equilibrium in 4 hours at 37.degree. C. Binding was specific and saturable with an apparent dissociation constant of 7.6.times.10.sup.-9 M. Decorin was shown to bind pepsin-derived fragments containing the collagenous domain of C1q and collagenase-derived fragments containing the globular domain of C1q. Since these fragments share a short sequence of amino acids, this finding suggests that decorin binds to a region of C1q located near the junction of the two domains.