Cell-associated heparan sulfate (HS) is endowed with the remarkable ability to bind numerous proteins. As such, it represents a unique system that integrates signaling from circulating ligands with cellular receptors. This polysaccharide is extraordinary complex, and examples that define the structure-function relationship of HS are limited. In particular, it remains difficult to understand the structures by which HS interact with proteins. Among them, interferon-gamma (IFNgamma), a dimeric cytokine, binds to a complex oligosaccharide motif encompassing a N-acetylated glucosamine-rich domain and two highly sulfated sequences, each of which binds to one IFNgamma monomer. Based on this template, we have synthesized a set of glycoconjugate mimetics and evaluated their ability to interact with IFNgamma. One of these molecules, composed of two authentic N-sulfated octasaccharides linked to each other through a 50-Angstroms-long spacer termed 2O(10), displays high affinity for the cytokine and inhibits IFNgamma-HS binding with an IC(50) of 35-40 nm. Interestingly, this molecule also inhibits the binding of IFNgamma to its cellular receptor. Thus, in addition to its ability to delocalize the cytokine from cell surface-associated HS, this compound has direct anti-IFNgamma activity. Altogether, our results represent the first synthetic HS-like molecule that targets a cytokine, strongly validating the HS structural determinants for IFNgamma recognition, providing a new strategy to inhibit IFNgamma in a number of diseases in which the cytokine has been identified as a target, and reinforcing the view that it is possible to create"tailor-made"sequences based on the HS template to isolate therapeutic activities.