A lectin highly reactive with dermatan sulfate (DS-lectin) was purified from adult chicken liver by gel filtration on Toyopearl HW-55 and subsequent affinity chromatography on new adsorbents which were prepared by immobilizing heparin or dermatan sulfate via the reducing ends on hydrazino-Toyopearl. The DS-lectin behaved as a single protein on polyacrylamide gel electrophoresis. On excitation at 280 nm, the DS-lectin emitted fluorescence centered at 336 nm, which was attributable to tryptophan residues and could be quenched by the addition of specific saccharides. The affinity constants of the DS-lectin with specific saccharides were calculated from the changes in intensities of fluorescence-difference spectra induced by the saccharides. Dermatan sulfate and protuberic acid, which is composed of L-iduronic acid and D-glucuronic acid (1:2), had the highest affinity constants among the polysaccharides tested. Partially N-desulfated heparin had a higher affinity constant than that of native heparin while dextran sulfate showed no affinity. D-Glucuronic acid and N-acetylneuraminic acid induced weak but significant quenching, but not N-acetylgalactosamine or cellobiose. These results were essentially in good agreement with those of hemagglutination inhibition tests and indicated that DS-lectin has a strong affinity for L-iduronic acid residues and probably carboxyl groups in the saccharides, while sulfate groups on the saccharides interfere with the specific interaction.