We have shown that thrombospondin (tsp), like fibronectin (fn) and von Willebrand factor (vWf), exhibits a rapid, specific and saturable binding to collagen type I fibres (from bovine tendon). The level of binding at saturation is very similar to that of vWf. As with fn and vWf, the interaction is ionic in character and appears to occur by a polyvalent mechanism since there is little inhibition of interaction by monomeric collagen. The conformation of tsp, like that of fn and vWf, is important since denaturation causes reduced complexing. Furthermore, conformational changes in tsp due to the presence of Ca++ can modulate the amount of complex formed under physiological conditions. Tsp, like vWf but in contrast to fn, shows little affinity for denatured fibres emphasizing the importance of collagen conformation. Pepsin digestion suggests an important role for collagen telopeptides; vWf- and fn-binding sites are located more within the collagen triple helix. Comparison of the effect on binding after leucine aminopeptidase or carboxypeptidase digestion suggests involvement of the N- rather than C-terminal telopeptides. No evidence was found for a role for fn, the proteoglycan PG2 or collagen type V, which could be present in type I fibres, in mediating the interaction between tsp and the fibres. VWf did not inhibit the interaction of tsp, but fn did slightly when tested in large excess. This suggests separate binding sites in collagen for all three ligands since fn and vWf are also known to bind independently of each other.