In order to clarify the mechanism of interaction between FGF-2 and heparin, the association structures between human FGF-2 and different kinds of regioselectively desulfated heparins were observed by small angle X-ray scattering. In the FGF-2-native heparin complex, the global FGF-2 molecules appeared to attach along heparin chain as strained unilaterally. The complexes with the 6-O-, or N-desulfated heparin seemed to have randomly associated structure as compared with above system. On the other hand, 2-O-desulfated heparin did not indicate the aggregation with FGF-2, indicating that the sulfate groups at O-2 of iduronate residues in heparin is most essential for association with FGF-2. These structural characteristics will be deeply related with signal transduction in the association with FGF-2 receptor. (c) 2004 Elsevier B.V. All rights reserved.