Heparan sulfate (HS) and heparin bind to virtually all chemokines and have been shown to play critical roles in the regulation of their activities. However, both binding mechanisms and structural features involved in chemokine-HS interactions remain poorly defined. In the study presented here, we analyzed the binding of heparin to RANTES(9-68), a N-terminally truncated form of the CC-chemokine RANTES. Using biochemical and surface plasmon resonance (BIAcore system) approaches, we showed that the RANTES(9-68)-heparin interaction was characterized by a complex binding model that involved dimerization of the chemokine through a mechanism of positive cooperativity. Since RANTES(9-68) remains monomeric in solution, we concluded that heparin induced chemokine dimerization. The structure of a complex involving a RANTES dimer and a heparin heptadecasaccharide was proposed by molecular modeling. This model was used to design a dimer of "head to head" coupled octasaccharides that would fit the internal symmetry of the chemokine dimer. This engineered oligosaccharide bound RANTES(9-68) much better than a natural heparin fragment of the same length, further supporting the interaction process and the proposed structural model. Altogether, the data reported here provide a basis for understanding the mechanisms by which HS modulates RANTES functions.