Heparan sulfates (HS) play an important role in the control of cell growth and differentiation by virtue of their ability to modulate the activities of heparin-binding growth factors, an issue that is particularly well studied for fibroblast growth factors (FGFs). HS/heparin co-ordinate the interaction of FGFs with their receptors (FGFRs) and are thought to play a critical role in receptor dimerization. Biochemical and crystallographic studies, conducted mainly with FGF-2 or FGF-1 and FGF receptors 1 and 2, suggests that an octasaccharide is the minimal length required for FGF- and FGFR-induced dimerization and subsequent activation. In addition, 6-O-sulfate groups are thought to be essential for binding of HS to FGFR and for receptor dimerization. We show here that oligosaccharides shorter than 8 sugar units support activation of FGFR2 IIIb by FGF-1 and interaction of FGFR4 with FGF-1. In contrast, only relatively long oligosaccharides supported receptor binding and activation in the FGF-1.FGFR1 or FGF-7.FGFR2 IIIb setting. In addition, both 6-O- and 2-O-desulfated heparin activated FGF-1 signaling via FGFR2 IIIb, whereas neither one stimulated FGF-1 signaling via FGFR1 or FGF-7 via FGFR2 IIIb. These findings indicate that the structure of HS required for activating FGFs is dictated by the specific FGF and FGFR combination. These different requirements may reflect the differences in the mode by which a given FGFR interacts with the various FGFs.