1. 1. 23Na + nuclear magnetic relaxation studies of cation binding to single-chain chondroitin 4-sulphate and dermatan sulphate and a multi-chain chondroitin 4-sulphate-polypeptide complex have been performed. Excess relaxation rates (longitudinal and transverse) for the bound sodium have been measured during titration experiments. The data were used to calculate the correlation time and the fraction of bound ions. This approach was also used to analyze the relative affinities of Na +, K + and Ca 2+. 2. 2. The excess transverse relaxation rates for 23Na + (obtained from line-width measurements or the decay of transverse magnetization) in solutions of single-chain and multi-chain chondroitin sulphate were recorded as a function of the degree of neutralization. There was a steep increase in transverse relaxation rate for the multi-chain preparation around pH 7 which was not seen with single chains. This could be ascribed to an increase in the correlation time for bound sodium. The alteration in binding properties observed with multi-chained material around pH 7 suggested a change in the quadropolar coupling constant for 23Na in this range. The behaviour of a multi-chained specimen (as expressed in excess line-width) during titration gradually changed to that of single chains upon prolonged exposure to alkali. 3. 3. Competition experiments using KCl additions to multi-chained sodium chondroitin sulphate at pH 12.3 and 3.3 indicated that Na + and K + have very similar affinities at high pH. However, at low pH there was an increased affinity for K + (expressed as a faster decrease in excess relaxation rate when KCl was added). 4. 4. Competition experiments using multi-chain chondroitin sulphate and Ca 2+ at pH 11.4 showed increasing excess relaxation rates during the initial phase. Above 4 mM Ca 2+ simple exchange of bound Na + with Ca 2+ was observed. The former behaviour was not seen at pH 3.3 nor with single-chain chondroitin sul[hate at pH 11.4. It is suggested that endogeneous Ca 2+ present in the multi-chain preparation is responsible for the pH dependence. The increased excess transverse relaxation rate seen with the multi-chained specimen upon Ca 2+ additions was referred to an increase in the correlation time for bound Na +. 5. 5. The most likely explanation for the abrupt increase in correlation time for Na + bound to multi-chain chondroitin sulphate around pH 7 is an aggregation between the side-chains. In the presence of Ca 2+, cross-linking of chains (clustered together on the protein core) was promoted and the local mobility of the Na + ions was further reduced. In addition to the formation of Ca 2+ bridges between chains, the results also imply increased binding of counter ions owing to a higher local charge density.