The interaction of basic fibroblast growth factor (bFGF) with heparan sulfate (HS)/heparin has been shown to strongly enhance the activity of the growth factor although the mechanism of activation is unclear. We have addressed the issue of the minimal stoichiometry of an active HS oligosaccharide.bFGF complex by chemically cross-linking the two components to form novel covalent conjugates. The cross-linking procedure produced both monomeric and dimeric bFGF. oligosaccharide complexes, which were purified to homogeneity. Dimer conjugates were shown to have been formed as a result of disulfide bridging of monomer conjugates. These monomer conjugates were subsequently found to be biologically active in a mitogenesis assay. We therefore conclude that a monomeric bFGF.oligosaccharide complex is the minimal functional unit required for mitogenic stimulation.