Binding of poly(amido amine) dendrimer to sodium hyaluronate (NaHA) in aqueous 0.25 M NaCl solution has been investigated by static light scattering. It was observed that the apparent weight-average molecular weight and the radius of gyration increase with the ratio of NH2 terminal groups in the dendrimer to the carboxylate groups in the NaHA, [NH2]/[COO-]. Up to [NH2]/[COO-] = 31, the observed variation of molecular weight was reproduced by the "average binding" model, where an average number of dendrimers binds to each NaHA chain. Based on the "critical binding" model, the maximum number, n(max), of dendrimers which can bind to a NaHA chain was calculated to be nmax = 300 for a solution of [NH2]/[COO-] = 56. The obtained value corresponds to the binding of one dendrimer per 1.5 repeating units on a NaHA chain. It is suggested from the observed radius of gyration that, while the dendrimer-NaHA complexes of [NH2]/[COO-] up to 5 maintain a wormlike character similar to NaHA without bound dendrimers, those of [NH2]/[COO-] above 10 behave like rigid rods. It is concluded that the hydrogen-bonding interaction, besides the electrostatic interaction, should play an important role in the formation of the NaHA-dendrimer complexes. (C) 2003 Elsevier Science (USA). All rights reserved.