Carcinoscorpin, the sialic acid-binding lectin from the horseshoe crab Carcinoscorpius rotunda cauda, has been immobilized using Sepharose. The immobilized lectin is shown to resolve the isoenzymes of alkaline phosphatase from sheep brain based on the difference in their sialic acid contents. The nonionic detergent Triton X-100 does not interfere with the binding efficiency of the column up to a concentration of 1% when tested with '*'I-labeled fetuin. However, the use of hexadecyltrimethylammonium bromide produced a 45% inhibition of binding of the labeled fetuin. The efficiency of various saccharides to elute the bound fetuin from the matrix was determined. U-(N-Acetylneuraminyl)
(2 -+ 6) 2-acetamido-2-deoxygalactitol was shown to be the most powerful agent in competing with the fetuin-lectin interaction. Moreover, o-glucuronic acid was also found to elute the bound fetuin from the immobilized lectin. "'I-Labeled fetuin in which an artificial heterogeneity is created by partial desialylation with neuraminidase resolved into three peaks using the immobilized lectin and a gradient of the disaccharide. It is suggested that this immobilized lectin could be used in the purification and resolution of minute amounts of several sialoglycoproteins.