The biological activity of basic fibroblast growth factor (bFGF) is influenced greatly by direct binding to heparin and heparan sulphate (HS). Heparin-derived oligosaccharides have been utilized to determine the structural requirements present in the polymer that account for binding to bFGF. We had previously demonstrated that fragments > 6 mer can inhibit the interaction between cell surface heparan sulphate proteoglycan (HSPG) and bFGF, and bFGF-induced proliferation of adrenocortical endothelial (ACE) cells. In contrast, oligosaccharides > 10 mer can enhance the binding of bFGF to its high-affinity receptor or support bFGF-induced mitogenesis in ACE cells (Ishihara et al., J. Biol. Chem., 268, 4675-4683, 1993). We have extended these studies to size- and structure-defined oligosaccharides from heparin, 2-O-desulphated (2-O-DS-) heparin, 6-O-desulphated (6-O-DS-) heparin, carboxy-reduced (CR-) heparin and carboxy-amidomethylsulphonated (AMS-) heparin. Oligosaccharides from these polymers were fractionated on a bFGF-affinity column and were assessed as inhibitors or enhancers of specific bFGF-derived biological activities. The results of these studies indicate that both 2-O-sulphate and the negative charge of the carboxy group [L-iduronic acid (IdoA) residues] are required for specific interactions of heparin-derived oligosaccharides with bFGF and for modulation of bFGF mitogenic activity. In addition, the charge of the carboxy groups in uronic acids can be replaced by other functional groups with a negative charge, such as the amidomethyl sulphonate moiety described here.