Various sulfated polysaccharides exhibit antiviral activity in vitro against certain viruses, including HIV,via an unknown mechanism. It has been known that the polysaccharides appear to bind strongly to the HIV virions and inhibit their association with the CD4 receptor of T lymphocytes. Ionic interactions are assumed to play a major role, as has been demonstrated in the interaction between a negatively charged polysaccharide portion of heparin and three lysine residues in antithrombin-III, A model system for interaction was constructed using sodium curdlan sulfate, a sulfated polysaccharide with extremely potent in vitro inhibition against HIV (abbreviated "CS"), and poly-L-lysine . HBr (abbreviated "PL") to simulate the alpha-6 helix of HIV gp120. The oligomeric nature of both species allowed characterization by H-1 and C-13 NMR. Upon mixing certain ratios of the two compounds in aqueous solution, a gel formed in the sample tube. The gel isolated was found to be composed primarily of poly-L-lysine and curdlan sulfate, having excluded most of the sodium and bromide ions. NMR analyses revealed a single species that is quite different from either of the two original species. The extent of gelling was affected by pH, average M(w) of the PL, temperature, and absolute concentration. Maximal gelling was attained at pH 4.0 similar to 8.4, 10000 Da, and 37 degrees C, respectively. It was concluded that CS can bind strongly with PL via ionic interactions, in such a way as to produce a polyion complex with a conformation distinct from either of the two original compounds. It was suggested that this interaction may help to explain the observed inhibition of HIV and other viruses by CS, in that CS may bind to highly basic helical segments of viral proteins and in so doing induce a conformational change that affects their functions.