Previous studies on the interaction of fibronectin with C1q have yielded apparently conflicting results since both the globular head regions (produced by collagenase digestion) and the collagen-like domains (produced by limited pepsin digestion) have been reported to bind to fibronectin. In this study, the binding of 125I-labelled fibronectin to either intact C1q, or the collagenase or pepsin digestion products, immobilised on plastic microtitre plates was examined. Inhibition of the C1q-fibronectin interaction by the C1q digestion products was also examined. The results confirmed that both globular 'head' region preparations and collagen-like region preparations, can interact with fibronectin. Since the fragments used in these studies share a section of common amino acid sequence from the C1q molecule it can be concluded that the binding site, on C1q for fibronectin, is located in a region formed from the residues 81-97 of each of the three chains of the C1q molecule.