Structural changes of the complex formed by the polysaccharide hyaluronic acid, as its salt, partially modified by a covalently bound fluorescence probe, 4-fluoro-7-nitrobenz-2-oxa-1,3-diazole, and dodecyltrimethylammonium bromide, were observed by measuring fluorescence depolarization with 0.175 M NaBr as a function of surfactant concentration. Values of depolarization change significantly between 2.0 and 10.0 mM surfactant. The critical micelle concentrations of the surfactant in 0.175 M NaBr in the absence and presence of the polymer are in the surfactant concentration range at which fluorescence polarization changes. Formation of the surfactant-polymer complex at high salt concentration is consistent with appearance of free micelles, which associate with the anionic polymer. (C) 1994 Academic Press, Inc.