Glycosaminoglycan (GAG)-collagen interactions play important roles in cell adhesion and extracellular matrix assembly; however, the chemical bases for these interactions are not fully understood. We have used affinity co-electrophoresis (ACE) (Lee, M.K. and Lander, A.D., Proc. Natl. Acad. Sci, USA, 88, 2768-2772, 1991) to study the binding of the heparan sulphate proteoglycan syndecan-1 and heparin to human collagens. [35S]Syndecan-1 [from normal murine mammary gland (NMuMG) epithelial cells] and low-M(r) (approximately 6 kDa) [125I]heparin were subjected to electrophoresis through agarose gel lanes containing human collagens at various concentrations, and binding affinities were measured from shifts in migration of the labelled materials. Results demonstrate that the affinities of each collagen for syndecan-1 and low-M(r) heparin were similar, and followed the order: type V >> type IV approximately type III approximately type I > type VI >> type II, and ranged in Kd from approximately 10(-8) to approximately 3 x 10(-6) M. These data suggest that syndecan-1 and heparin may contain similar collagen-binding determinants. It was also found that the same heparin subpopulation was selectively bound with high affinity by each of the collagens. The published amino acid sequences of the six collagens were examined for what are thought to be heparin-binding consensus sequences (Cardin, A.D. and Weintraub, H.J.R., Arteriosclerosis, 9, 21-32, 1989). The presence of such sequences did not correlate with affinity for heparin or syndecan-1, and collagens I, II and III lacked such sequences entirely. The data suggest that collagens may use novel types of binding sites to interact with GAGs.