The binding isotherms of C.I. Basic Red 18 with carboxy-methyl cellulose having various degrees of substitution have been determined in an acetate buffer at pH 4.2 and 30°C by a dynamic dialysis technique. The isotherms show a sigmoidal increase of binding with increasing free dye concentration in solution. The results strongly suggest a cooperative mode of binding. In the interpretation of the results, McGhee and von Hippel's theory considering cooperativity in binding was used. The intrinsic binding constant K, the number of consecutive polymer residues occupied by a single dye molecule n, and the cooperativity parameter were determined. A molecular mechanism of binding is proposed.