A lectin (LFA) which is highly specific for sialic acid has been purified from the slug Limax flavus by a combination of ammonium sulfate fractionation and affinity chromatography on bovine submaxillary mucin coupled to Sepharose 4B. The affinity-purified lectin appeared homogeneous by electrophoresis in the presence of sodium dodecyl sulfate. Below 1 mg/ml at pH 7, LFA exists as a species of Mr = 44,000 which is composed of two equal sized subunits. Above 1 mg/ml, the protein solution was observed to behave as a rapidly associating-dissociating system. N-acetylneuraminic acid and N-glycolylneuraminic acid gave a 50% inhibition of agglutination of erythrocytes by LFA at 0.13 and 0.81 mM, respectively. Galactose, N-acetylgalactosamine, galactosamine, glucose, N-acetylglucosamine, glucosamine, mannose, arabinose, xylose, fucose, glucuronic acid, alpha-methyl-D-glucoside, alpha-methyl-D-mannoside, lactose, and sucrose were ineffective inhibitors at concentrations up to 10-25 mM. Bovine submaxillary mucin, a sialoprotein, was a potent inhibitor of hemagglutination by LFA. Upon treatment of the mucin with neuraminidase, loss of inhibitory activity was observed which was proportional to the loss of sialic acid from the mucin.